A detailed chronological summary of the life of Koichi Hirose.
Background
Up until the events of the manga, Koichi has lived an ordinary life as a Japanese boy. It is known that he was born at the Red Cross Hospital in S City on March 28, 1984 at 6:27 AM. He was born prematurely, only weighting 2325g and kept in intensive care for 7 days.
Diamond is Unbreakable (1999)
Koichi meets the Joestars
Koichi is a first-year student in class 1-B of Budogaoka High School. On the first day of school, he bumps into Jotaro Kujo who manages to catch him as well as all his scattered item immediately (Star Platinum is responsible for it), which surprises him. Jotaro asks him about a certain "Higashikata" and if he knows how to reach their address. A group of thugs pass by and angrily demand Koichi salutes then. Afraid, Koichi does so. The thugs decide to victimize another first-year Josuke Higashikata and Josuke terrorizes them. Koichi witnesses Jotaro and Josuke's presentation, a subsequent scuffle, and is informed of a criminal named Anjuro Katagiri menacing the town. Josuke and Koichi become comrades. Later this day, Koichi witnesses a hostage situation where Josuke intervenes and where they discover Aqua Necklace.
The Nijimura Brothers
After Angelo is defeated by Josuke, Koichi and him stumble upon an abandoned house on their way back from school. Having seemingly seen a mysterious silhouette at one of the windows, Koichi decides to stick his head through the front gate to take a closer look. Suddenly, a student emerges from the house and slams the gate on his neck. Koichi is shot with the Bow and Arrow by Keicho Nijimura, who was searching for a Stand capable of killing his father. However, the arrow fatally pierces Koichi's flesh, causing him to quickly fall unconscious. Okuyasu Nijimura stops Josuke from healing Koichi and fights him with his Stand, The Hand, which can erase space between objects.
Josuke defeats Okuyasu but Koichi is dragged inside of the house by Keicho. Koichi is left in the middle of a dark room. Feeling that he owes Josuke for healing him, the younger Nijimura brother then uses The Hand to teleport Koichi toward Josuke, and departs. Josuke heals Koichi and then gets attacked by Keicho's Stand - Bad Company - which is comprised of a miniature army, complete with tanks and assault choppers. Koichi is now able to see Bad Company, meaning he has become a Stand user too. Keicho reveals himself as he's interested in Koichi's ability, and to force him to summon his Stand, has him stabbed by a miniature Green Beret. Koichi's Stand is revealed to be an egg and Keicho, satisfied, proceeds to separate Josuke and Koichi with a barrage. Josuke manages to defeat Keicho.
At Koichi's initiative, the duo keep looking for Keicho's Arrow. Koichi reaches the attic but the two hear strange noises. When he tries to kick the door, Koichi is grabbed by a monster, which is revealed to be Keicho's mutated father. Keicho comes and reveals his tragic story. Josuke manages to prove that there is some humanity left in him but Keicho refuses to hand the Arrow. Koichi witnesses the theft of the Arrow by Red Hot Chili Pepper and Keicho's murder by the same Stand.
Meeting Tamami Kobayashi
Koichi is gleefully riding on his new bike to school when suddenly he runs over a strange bag then hears faint meows and sees a dark liquid oozing from the bag. Koichi understands that he has just run over a cat and a stranger named Tamami Kobayashi tries to extort 500,000 yen out of Koichi with his Stand, The Lock, which works on guilt, and will automatically grow when the victim feels guilty. Suddenly, Josuke and Okuyasu who have seen the two from afar appear to rescue Koichi. While Tamami manages to make Okuyasu feel guilty after punching him, Josuke uncovers a toy cat from the bag and heals Tamami with Crazy Diamond, exposing his schemes and making The Locks disappear from Koichi and Okuyasu's chests. However, Tamami flees and steals Koichi's 7,000 yen. Around 4:07 PM, Koichi comes back home and sees with shock that Tamami is massaging his mother's shoulder and chatting with her.
Screaming at Tamami what he is doing here, Koichi is presented the 7,000 yen Tamami stole from him and is now pretending to return. Tamami adds that when Koichi bumped into him with his bicycle, they exchanged their wallet, and Tamami claims that his contained over 500,000 yen. Tamami successfully makes Koichi's mother feel guilty and The Lock appears on her. When Koichi's sister appears with The Lock on her chest too, this enrages Koichi who pushes his mother and sister out of the room and summons his Stand, which hatches Echoes. Echoes is incredibly weak, but instead creates several loud noises by planting sounds on Tamami, which drive him crazy. However, Tamami refuses to surrender and stabs himself, throwing the knife toward Koichi. When Koichi's family barges into the family room, they see Koichi having seemingly stabbed Tamami. The guilt makes Koichi's sister faint and his mother try to kill herself, but Koichi uses the sound "Believe me!" to relieve his mother of the guilt. His Stand defeated, Tamami yields and begs a more confident and assertive Koichi to spare him.
Toshikazu Hazamada
Tamami meets Josuke and Koichi at their school to inform them of another Stand user in town: Toshikazu Hazamada, a student at the same school as Josuke, and tells them what happened to one of Hazamada's classmates. Intrigued, Josuke decides to investigate Hazamada accompanied by Koichi. Josuke and Koichi head first to Hazamada's classroom where they do not find him, but are noticed by him nonetheless. They then investigate his locker, where a doll which transforms into Josuke as soon as he touches it attacks them. Surface, which is able to transform into someone and control their movements as long as they are in eye contact, takes out Koichi by having Josuke violently elbow him, and effortlessly evades Crazy Diamond's attacks. It then tosses a pen to Josuke and forces him to pierce his eye with it. Hazamada then reveals himself and, satisfied, decides to go for Jotaro. However, Echoes mimicked the sound of Josuke's eye being pierced, fooling Hazamada, as Josuke secretly healed Koichi when he elbowed him. They then call Jotaro's hotel room, but Surface has already called a meeting for Jotaro.
While Josuke and Koichi pursue Hazamada, the latter is being distracted by the numerous girls approaching Surface, mistaking it for Josuke. He then dirties a motorcycle, antagonizing two bikers. However, Surface quickly takes out one of them and holds the second. Hazamada then threatens to cut the biker's mouth with a box cutter but is interrupted when a glass shard is thrown toward him. Surface catches the shard, and Josuke repairs the shard into a bottle which cuts off Surface's right hand, then leaves. Realizing that his plan is in danger, Hazamada decides to hurry toward his meeting place with Jotaro. At a railway, he hears the siren announcing an oncoming train and decides to take a detour, but in fact, has been tricked by Echoes's ability. Josuke and Koichi manage to meet Jotaro first and warn him about Hazamada and Surface, but suddenly, Josuke takes a pen out of Jotaro's coat. He sees Surface controlling his movement from behind a window, and almost stabs Jotaro in the neck. Fortunately the two bikers Hazamada had antagonized earlier knock him out and drag him to be beaten elsewhere, saving Jotaro. Meanwhile, Josuke takes out his frustration on the doll.
Yukako Yamagishi Falls in Love
Koichi is invited by a fellow student, Yukako Yamagishi, to drink in a café. Curious, Koichi asks what Yukako wants and she confesses her love to Koichi, shocking the boy. However, Koichi's joy at being proposed to is cut short when Koichi doesn't answer clearly to how much he reciprocates Yukako's love, which makes Yukako enter a brief state of rage before calming herself and leaving in shame. Abashed by Yukako's temper, Koichi sips his cola and realizes that it is full of hair.
The next day Yukako tries again to approach Koichi and goes overboard with her display of affection, thus Koichi uses his class representative as an excuse for fleeing. Later Yukako jealously plants some of her hair on the representative, which results in the girl gagged and blindfolded by hair as her own hair burns. Fortunately, Okuyasu uses The Hand to erase the fire and some of the representative's hair. Josuke and Okuyasu realize that Yukako is up to no good.
Later Koichi, Josuke, and Okuyasu meet to discuss what to do with Yukako. Stressed by Yukako, Koichi's English grades are dropping, and he wants to stop Yukako. Unwilling to incur her wrath, the trio eventually settles for indirectly discouraging Yukako from seducing Koichi by ruining his reputation. Josuke and Okuyasu proceed to loudly slander Koichi near Yukako, aware that she is listening. However the same night, Yukako kidnaps Koichi, wanting to work for his "betterment." Later, Koichi finds himself in a villa and unable to contact the outside world. He's realized that Yukako is a Stand user, but has the advantage of surprise, so decides to play along. While Yukako prepared a copious breakfast, she forces Koichi to attend an English quiz where a false answer results in being force-fed things such as soap or asparagus rolled in dictionary pages. Koichi interrupts the quizz and tries to flee, but Yukako effortlessly restrains him as she planted some of her hair on Koichi, and can lift him up to the ceiling. Yukako leaves, assuring that no one will come to the villa until summer and that they will have time to "make progress".
Koichi cannot open the lock to the bathroom, causing him to wet himself. Ashamed, he tries to dissuade Yukako from liking him but it fails as she still sees something shining within him and even says she is happy to do his laundry. She shows him the electric chair she is building for his dinner study time, causing him to worry about what will become of him.
She goes out to order olive oil and Koichi reasons there must be a pay phone nearby, which is luckily within 48 meters (Echoes has a range of 50). Unfortunately, he does not have a 10 yen coin to pay for the call to Josuke and reasons that if he calls the police Yukako would simply kill them. Yukako approaches Koichi and figures out he knows about the pay phone but warns him he cannot escape. As she goes to the pay phone to order, Koichi manages to use Echoes to make the calling noises of the phone (so long as the phone hears the sound of the button the call will go through) and manages to call Josuke as he and Okuyasu are trying to find his location.
Yukako approaches the house as Koichi barricades the door with a shoe cabinet and the electric chair from earlier while nailing them to the door. While Yukako is happy that the two have powers as it further deepens the bond between them, Koichi uses Echoes to produce the sound "I hate you" from all over her body. Yukako is so stubborn in her delusions that the sound has no effect. She tries to open the door but is shocked by the doorknob, which was rigged with the electric chair. Angry at this act of defiance, Yukako manipulates the hair she implanted in Koichi's scalp to thrust him against the door to open it. She uses her Stand to surround the house in hair and prepares to attack. However, due to the stress Koichi's Stand "dies". Koichi mourns for Echoes, as he believes he pushed his Stand too hard and killed it, but Yukako manages to see that the Stand is not dying but was evolving and worried, moves the shoe cabinet away from the door and angrily asks Koichi what is happening to his Stand.
Koichi then realizes that his Stand is not dying but has evolved into Echoes ACT2 and Yukako is suddenly blown away from the door. Koichi notices sound effects on the door and realizes he was hoping for her to be blown away and since the door has the words blown away on it she flew. Yukako proclaims Koichi is hers approaches the house once more and sees that Koichi has cut off his hair in order to remove the hair she implanted in his scalp. He warns her not to come closer and that she can't beat him, causing her to become enraged as she believes he is getting too cocky and that he belongs to her and shouldn't defy her. She comes to the house but burns her hand on the stair rail as Koichi implanted the word sizzle on it, causing the rail to feel hot.
Yukako becomes even more enraged and threatens to kill Koichi if he does not say "I love you" to her. He refuses and once more pronounces his hatred of her. She uses her hair Stand to grab onto Koichi, only to realize he implanted words on himself, causing Yukako to once more fly away. The stress of the battle causes her hair to turn white and Koichi approaches her to make sure she is alive. While he is relieved she didn't die, Yukako awakens and captures Koichi's Stand, threatening to tear it apart since she can't touch Koichi herself, slicing part of Echoes's tail and throwing it in the ocean. However, Koichi warns her the ground is unstable as Echoes not only heard Yukako's heartbeat as he approached her to ensure she was alive but also heard the ground giving out.
Yukako and Echoes fall and as she is about to be pierced by a rock, Yukako is bounced safely to the surface. She realizes that as she was trying to kill him that Koichi had already come up with a plan to ensure she would be saved and implanted the "Boing" sound when she tore off part of Echoes tail and threw it near the rock. Koichi mentions how she didn't listen to him and leaves while Yukako realizes she lost from the beginning and is even happier than before as she loves him more than ever and doesn't care if he doesn't notice her as she is happy just thinking of him. Josuke and Okuyasu catch up to him after his fight and ensure he is okay. They also notice Yukako with her white hair and that she is smiling while looking in their direction causing them to panic and try to leave. Meanwhile, some fishermen who saw Yukako fall and bounce right back up name the area Boyoyoing Cape, with a rumor going around that a girl who tried to commit suicide was bounced right back up and became a popular fisherman spot. Some time later, Koichi is given another English test, having this time scored a perfect grade, yet he is unsure about how to feel about this.
Red Hot Chili Pepper
Akira Otoishi reveals later that Koichi was one of the Stand users he watched with his Red Hot Chili Pepper. Koichi is summoned to an empty field with Josuke and Okuyasu, where Jotaro explains to them that Joseph Joestar is coming to Morioh and help find the user. However, Red Hot Chili Pepper comes out of Okuyasu's motorbike and manages to flee despite Okuyasu's efforts.
Later, Koichi and Josuke are tasked with guarding the port while Jotaro and Okuyasu go join Joseph's boat. Koichi bears witness to Akira Otoishi's appearance and the subsequent fight with Josuke, which Josuke wins. Although Akira manages to infiltrate the boat, Okuyasu luckily stops him. Koichi is happy to see Josuke reunites with his estranged father.
Koichi & Rohan Kishibe
Koichi meets Hazamada in the streets of Morioh. Despite an awkward conversation as former enemies, Hazamada and Koichi quickly bond over their love of manga. Hazamada then points out that the famed manga artist Rohan Kishibe has recently moved in the town, and the two head to his house in the hopes of meeting him. As Koichi tries to ring the bell, Rohan grabs his arm from behind the door, scaring the boy a little. However, the mangake is happy to learn they are fans and gives them a tour of his studio.
In his office, Koichi and Hazamada are impressed that Rohan is talented enough to draw his manga alone but are creeped out by how far Rohan goes to perfect his manga as he tastes a spider right in front of them. Rohan leaves and Koichi and Hazamada can't resist the chance to sneak a peek at his new manuscript. The moment they do, however, they fall under the power of Rohan's Stand, Heaven's Door, which allows him to open their bodies like a book and read all of their secrets.
Koichi summons his Echoes but the attack misses as Rohan can also write commands on those he turns into books, thus he's prevented Koichi from attacking him as a precaution. Highly interested in Koichi's personality and colorful life, Rohan proceeds to take pages from Koichi. At the end, Rohan lets the two go home with no memory of what just happened. That night, Koichi senses something amiss when he discovers he's lost 20 kg, but suddenly forgets about it just as he prepares to inform Josuke. Under Heaven's Door's influence, Koichi goes back to Rohan's house the next day, visiting Rohan in his studio. Koichi then sees Rohan finishing drawing a page at astonishing speed, as Rohan was made enthusiastic by reading Koichi's files. Wanting to use every bit of Koichi's memories, the mangaka then rips off yet another page out of Koichi, making him remember that Rohan is a Stand user.
As Rohan plans to keep taking pages from Koichi to inspire his manga, the two hear a doorbell ring. It is Josuke and Okuyasu and Koichi slips away to ask for their help. Unfortunately, the safety lock written inside Koichi makes him unable to warn them and the duo seemingly leaves. However, after Koichi goes back up to Rohan's studio, Okuyasu barges in from a window, as the duo saw that something was wrong after seeing Koichi's untreated cut on his hand. Koichi is subsequently saved by Josuke, who beats up Rohan in an uncharacteristic rampage as Rohan insulted his hair style.
Rohan Kishibe's Adventure
A month later, Rohan, who has recovered from his injuries from Josuke, meets Koichi. The mangaka enlists him to find out about the strange alleyway that isn't on the map of Morioh.
They enter the strangely abandoned alley but they keep finding themselves on the same street with a mailbox. They are then approached by a girl named Reimi Sugimoto. Rohan immediately uses Heaven's Door against her. To his surprise, she turns out to be a regular girl as he keeps reading more about her character, while Koichi scolds Rohan for reading far too much of her personal information. After writing down a command for her to forget the ordeal, Reimi then interacts with the two, much to Rohan's displeasure.
Reimi narrates the story of a girl and her dog being assassinated and then reveals that both she and her dog Arnold are the ghosts of the story's victims. This also reveals that the Ghost Alley is a limbo between the World of the Living and Heaven. Reimi informs the others that the man who murdered her has been secretly killing others and asks them to stop him. After they agree, Reimi points them towards the exit, warning them not to turn around too soon as they will be pulled into the afterlife by dark spirits.
Rohan and Koichi feel and hear the malicious spirits behind them as they approach the exit. Losing his calm, Koichi tries to run to the exit and is told by Reimi's voice that it is safe to turn now. It is only the spirits tricking Koichi, and a myriad of hands come out of the darkness to grab Koichi and Echoes. Fortunately, Rohan manages to use Heaven's Door to get both him and Koichi to safety. Afterwards Koichi informs Josuke and Okuyasu about the murderer, but Josuke argues that there is no urgency to what seems a "mundane" crime involving no Stand user. Koichi is almost run over when he hastily crosses the street and apologizes to the driver, who is revealed to be Yoshikage Kira, the serial-killer in question.
Yukako Yamagishi Dreams of Cinderella
In the streets, Koichi meets Yukako, who has recently been beautified by Aya Tsuji and her Stand Cinderella. Koichi wishes to put aside their past fight and make peace, to Yukako's joy. Although Koichi and Yukako go as far as touching each other's hand, the time limit has passed. Koichi begins to have stomachaches and runs away. Koichi suddenly feels better, and noticing that he is holding Yukako's bag, runs back to give it to her. When he sees Yukako, Koichi's heart is enthralled by her beauty. Indeed, she has just revisited Aya Tsuji again to upgrade her treatment. However, Rohan appears and pressures him into following him. Yukako, who pictured both of them having an intimate moment, is taken aback. Finally, events conspire to make them hug each other and Joseph accidentally pushes Koichi in Yukako's arms; Yukako and Koichi share a passionate kiss.Several days later, Koichi informs Josuke and Okuyasu of his new relationship, which shocks them. However, he finds it strange that after their kiss, Yukako never contacted him again. In the streets, he sees a woman resembling Yukako, but the hideous woman denies being her. It is indeed Yukako, who crashes Aya's salon, ordering her to fix her face. However, Aya reveals that Yukako's face is gone forever and her features begin to progressively disappear as a result of tampering with Fate. Koichi suddenly enters the salon, having recognized Yukako by her personality alone. Impressed, Aya decides to give Yukako a second chance and spawns several faces, saying that she can choose whichever feels like hers, but choosing incorrectly would make her incredibly ugly forever.The truth is that had Aya lied, and that none of the faces belonged Yukako; only if she realizes this would Aya fix her face. Yukako is unable to decide, and asks Koichi to choose for her, saying that she will be satisfied with his choice, surprising Aya. Furthermore, when Koichi requests to be blinded in case Yukako ends up ugly to ease her pain, Aya is so moved by the couple's love for each other that she restores Yukako's features anyways. Everyone ends up happy.
Sheer Heart Attack
After Shigechi is killed by the murderer Yoshikage Kira, Josuke organizes a meeting consisting of himself, Okuyasu & his dad, Koichi, Jotaro, Joseph and Shizuka, Yukako, Hazamada, Tonio, Aya, and Rohan and brings them to meet Reimi. They are all informed about the killer and deduce that he must be a Stand user. The deduction prompts Jotaro to decide tracking down the culprit using his button which is the only clue Shigechi managed to give to Josuke before his death. While Josuke leaves, he briefly crosses paths with Kira.
Four days pass with no results before Koichi runs into Jotaro and accompanies him on his search for someone who may have repaired a jacket with similar buttons. The two find a shoe store that also specializes in tailoring. Jotaro admits that while he did check every tailor in Morioh, he did not consider a place like this, and goes in to try and find some leads. There, they learn that the store owner recognizes the button, pointing out the recently-repaired purple jacket that the button came from. However, the store owner decides to check the tag on the jacket to ensure that he's remembering the owner's name right, only to have his right hand suddenly blown off. As it turns out, Kira had been hiding in the back room and had deployed another bomb in the event that someone would use his lost button to track him down. Killer Queen's second bomb, a miniature tank-like object, rushes into the unconscious store owner's mouth before Kira grabs and shakes his jacket to bait Jotaro & Koichi within the bomb's range. Koichi runs up to try and take the coat back, but is pulled away by Jotaro as the bomb goes off.
Koichi notices that the killer is gone and rushes in to pursue. However, Jotaro interrupts him, explaining that the enemy wouldn't try to cut off loose ends, therefore they are still in danger. Despite Koichi's skepticism, Sheer Heart Attack shows up again and attacks Koichi. Star Platinum appears to defend Koichi and pummels the miniature tank, but the repeated punches barely dent it. While Jotaro fights Sheer Heart Attack, Koichi figures that such a powerful Stand means that the killer is nearby and defenseless, and proposes to send Echoes after him. Despite Jotaro's insistence that Koichi only defends himself with his Stand, Koichi sends him away to scout the area, only to be surprised that the enemy is already so far away from the store. Suddenly Sheer Heart Attack swifts his attack to Koichi again who is vulnerable. Thankfully, Jotaro stops time and starts a fire near him, having figured out that the enemy is heat-seeking. However, he cannot avoid the blast and is gravely wounded.
Koichi tries to wake Jotaro up while Sheer Heart Attack resumes his pursuit. When he drags Jotaro into the back of the store, Koichi sees the tank break all the lights along the way, allowing Koichi to discover the bomb's pattern. Now in the kitchen, Koichi sees that no immediate heat source is available, thus resorts to Echoes ACT2 to make a bait for the enemy out of a sizzling sound. During that short respite, he calls Josuke for help, but the stove he shut off earlier begins to heat anyway, making Sheer Heart Attack destroy it and explode. Koichi, who was near the center of the explosion, is propelled out of the store and wounded. At the last moment, Echoes evolves again into a new act, Echoes ACT3. ACT3 turns out to be not only more powerful than its previous iterations, but sentient as well. However, it is its power of making objects heavier which manages to neutralize Sheer Heart Attack, leaving the bomb trapped in the asphalt.
Koichi talks with Echoes ACT3 who explains that Sheer Heart Attack will only stay weighed down if it is within 5 meters of him. Koichi then sees Kira approaching him and deduces that he is the killer. Angry, he sends Echoes to punch Kira, but Killer Queen easily parries Echoes and sends it flying; Echoes lands outside of his 3-Freeze attack's range, releasing Sheer Heart Attack and Kira from its weight. Disconcerted at the sight of Kira seemingly having two Stands, Koichi is easily defeated when Killer Queen stomps Echoes on the floor. Kira, deducing that he has three minutes to kill Koichi, decides to pay him back his public humiliation with a severe beating. However, Koichi shatters Kira's confidence when he steals his wallet, learns all of his personal information from his driver's license, and exposes how easily Kira will be caught. Outraged, Kira makes Killer Queen punch through Koichi.
While Josuke and Okuyasu are still running, Kira tries to disintegrate Koichi but is distracted by Koichi's inside-out sock. After fixing it, he is interrupted by Jotaro, still heavily wounded, but whose Star Platinum is more than enough to pummel Kira and send him crashing into a garbage bin. Kira and Jotaro then both fall unconscious. When Josuke and Okuyasu arrive, Koichi is thankfully healed by Crazy Diamond.
Josuke and Okuyasu find Kira, who cuts his left hand and sics Sheer Heart Attack on them. Josuke, instead of fighting the sub-Stand, decides to repair Kira's hand so he can follow it back to its proprietor. The group eventually enters the Cinderella beauty salon where an injured Aya reveals that Kira forced her to switch his face with that of a random bystander. Aya explodes before revealing Kira's new identity, and the explosion, unfortunately, distracts the group, and they lose the hand within a crowd of commuters. Yoshikage Kira has escaped, having taken on someone else's face, clothes, and life.
Jotaro, Josuke, Koichi, and Okuyasu search Yoshikage Kira's old home looking for any noteworthy traits that they can use to track him down. A Polaroid camera suddenly snaps a picture of Jotaro and Josuke. They are attacked by the ghost Yoshihiro Kira, Yoshikage Kira's father. Yoshihiro traps Josuke and Jotaro in the photo, and also prevents Koichi and Okuyasu from entering the photographed space. Yoshihiro almost beheads the JoJos but Jotaro traps Yoshihiro alone in a photo before folding this new photo in two, taping it shut, and tacking it onto the wall.
The gang continues their search, but Yoshihiro manages to bluff Okuyasu into releasing him. Yoshihiro escapes, stealing the second Bow and Arrow from Josuke and fleeing successfully.
Highway Star
In an effort to hinder the heroes' investigations, Yoshihiro uses his Arrow to create as many Stand users as possible. One day, Koichi prepares to walk his dog Police, but he is suddenly called by Josuke who informs him of the Stand Highway Star. Koichi remembers a news story about a drunk driving accident in the tunnel where Highway Star first appeared; after some thinking, Josuke comes to the conclusion that this unknown biker must be the Stand user, and Highway Star is his means of speeding up his recovery. Josuke heads to the hospital where the user is at while continuing to evade Highway Star through various convoluted means.
Koichi reaches the hospital but is unable to find out anything about the biker because visiting hours are over. Desperate for information, Koichi sends out Echoes ACT3 to increase the mass of and tip over several expensive medicine bottles. The administrator ends up at Koichi's mercy and is forced to tell him that the man he is looking for is Yuya Fungami in Room 525, just in time for Josuke to come crashing in. Hearing this new information, Josuke rushes into the elevator, with Highway Star in hot pursuit. Since ACT3 is able to stop only one of the enemy Stand's feet at a time, it eventually catches up with Josuke and starts feeding off of him. However, Josuke still has enough strength to make it into the room and defeats Yuya.
Enigma Boy
One day, Koichi is attacked and turned into paper by Terunosuke Miyamoto. Josuke and Yuya look for him. Josuke is also imprisoned in paper but Yuya manages to free them from a shredder by letting himself get turned into paper. Koichi uses Echoes to weight Miyamoto's hands when he tries to take Yuya hostage and Josuke eventually turns the enemy into a book.
Cheap Trick
Rohan Kishibe becomes the unwilling host of Cheap Trick, and calls Koichi for help. When Koichi arrives at Rohan's house, the mangaka's claim of having an invisible Stand on his back that will kill him if he ever shows his back make no sense to Koichi and the boy thinks that Rohan is messing with him. Koichi leaves.
However, Koichi sees Rohan in the streets, still hiding his back. Knowing that the mangaka wouldn't be acting like this normally, he follows Rohan. When Rohan finds himself surrounded by hostile cats and dogs, Koichi comes to help repel the dogs. He applies Echoes's ability to Rohan's back but even this cannot help Rohan get rid of Cheap Trick. The mangaka seemingly shows his back to Koichi out of desperation but it is revealed that they are in Ghost Girl's Alley. When Cheap Trick turns back to attack Koichi, he is taken by the malicious spirits of the alley. Exiting the alley, Reimi, Rohan and Koichi notice Hayato Kawajiri's photo next to that of his father, Kosaku Kawajiri.
Crazy Diamond is Unbreakable
Koichi, Jotaro, Josuke, Okuyasu and Rohan agree to meet the next day at 8:30 to find and interrogate Hayato. Unbeknownst to them, Kira has developed a new power, Bites the Dust. If anyone interrogates Hayato, Killer Queen enters their eyes and detonates their bodies before rewinding time to one hour before. Koichi is killed in this same in one of the loops.
After Josuke, Hayato and Okuyasu cooperate to battle Kira, Koichi finally finds Yoshikage Kira again and is fundamental to his defeat, as he multiplies the weight of Kira's left hand before he can activate his Bites the Dust ability. As Kira is accidentally killed by an ambulance, Koichi looks away in horror. After Kira's death, he then sees off Reimi as she, along with her dog, goes to heaven.
He is seen hanging out with Yukako next to the Angelo rock and wants to eat a lunch she made and holds her hand as they walk. He is later seen talking to Josuke and Okuyasu about a rumor the latter heard about Rohan being caught shoplifting at Kame Yu.
Vento Aureo (2001)
Gold Experience
In 2001, Jotaro requests Koichi to go to Italy and find a certain "Haruno Shiobana" and take a skin sample of him. Noting a passing resemblance with Jotaro, Koichi is convinced when Jotaro tells him that the trip's spending will be taken care of by the Speedwagon Foundation.
Koichi Hirose arrives in Naples Airport, gets acquainted with the usual taxi fees to Naples, and prepares to take a taxi to his hotel. Exiting the airport, he coincidentally meets Giorno Giovanna, who proposes to ride him to his destination for an unusually small fee. Very suspicious but desperate to avoid the long queue, Koichi reluctantly accepts Giorno's proposition. Indeed, as soon as Koichi puts his luggage inside the car, Giorno tries to take off without the Japanese teen. While Koichi easily stops the car thanks to his Stand, Echoes ACT3, Giorno calmly flees with a smile on his face. Koichi then sees that his luggage has somehow disappeared, with a small frog appearing where it should have been before promptly leaping into a sewer grate. Hearing two guards chatting about Giorno, Koichi decides to ask them for further information.
Koichi overhears the nearby policemen talking about Giorno's failure and realizes that "Giorno Giovanna" is actually the "Haruno Shiobana" he'd been sent to find. He doesn't, however, figure out that the frog he found in the back of the car is actually his luggage.
Koichi walks the streets looking for Giorno, upset about his misfortune. By pure chance, he sees him at a café and furiously asks the boy for his passport and luggage. Giorno tries to run away, but Echoes multiplies Giorno's hand's weight. Cornered, Giorno is forced to summon his Gold Experience and creates a tree to lift him up to a roof. Koichi tries to break the tree, only for his attack to be reflected and knock him down. Before leaving, Giorno openly observes that Koichi has a merciful nature. Jotaro Kujo receives a call from Koichi, who nervously narrates his encounter with Haruno, revealed to be Giorno. Explaining everything he knows, Koichi is then told that Giorno is in fact the son of DIO and that Jotaro wanted someone reliable to stealthily investigate the boy.
Joining the Gang
Sometime later, when Giorno is undergoing the test to join Passione, Koichi locates his apartment and searches for his passport. He nearly extinguishes the lighter flame which Giorno had stood in a loaf of bread, but Giorno whisks it away with Gold Experience just in time. Having successfully found his passport, Koichi looks through the window and sees Giorno with the dead janitor. Unaware of the situation, he barges out of the dormitory but attracts Black Sabbath who attacks him with the Arrow. Giorno nonetheless saves him by turning the handrails into flowers and forcing Black Sabbath to flee.
During this reprieve, Giorno explains to Koichi that despite everything he is righteous at heart. Somewhat calmed, Koichi explains the use of the Arrow and that Black Sabbath must be an automatic Stand following preset instructions. Black Sabbath eventually outmaneuvers the two by using birds to move between shadows and grabs Giorno from the shadow of a nearby tree. However, Echoes then shatters tiles and Gold Experience overcharges the roots of the tree with life, making it wither and exposing Black Sabbath to the light. Now considerably weakened, Black Sabbath is pummeled into disappearing for good. Afterward, Koichi rushes to call Jotaro Kujo for help against the Arrow, but Giorno stops him. As Polpo is probably unaware of the battle, Giorno sees an opportunity to continue infiltrating the gang. Revealing his dream to Koichi, Giorno convinces him of his righteousness and to keep his secret. Later, Koichi says he will report to Jotaro and leave for good.
In the anime, Giorno trusts him enough to tell him everything. Koichi continues his trip and he gives the report to Jotaro when he gets home. It's implied Koichi stood up for him since Jotaro doesn't pursue Giorno anymore.
Spin-Offs
The Book ~ 4th another day
On January 4, 2000, Koichi and Rohan find a cat named Trinita covered in blood and the two try to find her owner. Koichi soon discovers that Hanae Orikasa, the cat's owner, was killed under mysterious circumstances and tags along with Rohan to find the killer. After taking Trinita from the police station via Echoes, they discover, with the help of Heaven's Door, that the killer might be a Stand user, and has a very distinct characteristic: red marks on his arms.
Koichi later works with Yukako to discover if the killer is from the same class as them, using his Echoes ACT2. Learning the fact that there are multiple students in the school with this feature, Koichi tries figuring out the secret inside this. After Koichi knows about Josuke's mother Tomoko getting attacked, he meets up one of his schoolmates with red marks on his arms, asks him about this, and gets surprised with his response that he actually did this by himself. Then Rohan comes and uses Heaven's Door to read the schoolmate's memory. By reading his memory, Rohan deducts the Stand ability of the culprit and erases those made-up memories. After this investigation, Koichi asks Rohan to erase the memories from Tomoko as well.
Later, in March 2000, with Rohan's investigation, Koichi and his fellows have more information about Hanae Orikasa and the culprit. When Koichi and Yukako are studying at Thorn Parlour (Morioh public library), they encounter Chiho Futaba. Koichi is a little confused to know that Chiho knows his name.
On a snowy day, Koichi goes home by bus after school and runs into Josuke. They talk a little about the pompadour young man who saved Josuke's life when he was 4. Then Koichi glances a slide. They run to the spot and find an orphanage. After they ask the staff and the dean, Josuke and Koichi are quite sure that Takuma Hasumi is the culprit they are chasing. After getting Takuma's address, Koichi and Josuke decide to go to his home the next day.
That night, Koichi is informed by Yukako that Chiho's home is on fire, so he immediately calls Josuke, who doesn't reply and is actually on his way to Takuma's home. Josuke calls back when he fails to find Takuma in his home, then Koichi tells Josuke that Takuma might be at Thorn Palace now. Finishing the phone call, Koichi starts walking to Thorn Palour. Koichi also calls Okuyasu and tells him to go to Thorn Parlour on his way. It's assumed that the battle between Josuke and Takuma is already over when Koichi eventually arrives Thorn Palour, and Koichi calls the ambulance and police anonymously.
In August 2000, Koichi comes across Chiho accidentally, who is now living with her mom outside Morioh. After some chatting, he is astonished to find out Chiho is pregnant with Takuma, her half brother's child. Deciding to keep this encounter as a secret, Koichi prays that Chiho and her child will have a good future when Chiho is leaving Morioh.
Rohan au Louvre
Koichi makes a brief appearance, hearing Rohan's story about Nanase Kishibe along with Okuyasu and Josuke. Koichi questions Okuyasu about his opinion on Rohan's likeness with Mona Lisa, as they have different genders.
Eyes of Heaven
The story begins as an ordinary day for Koichi, as Okuyasu has called in sick at school. Josuke and Koichi are heading to Okuyasu's residence to check on him though Josuke thinks Okuyasu is merely pretending to have a cold to skip school while Koichi is actually worried about his health. At this point, they see a younger version of Jotaro and wonder if it's a doppelganger. Their conversation is nonetheless interrupted when Okuyasu and Rohan Kishibe appear, both being inexplicably hostile and attacking the heroes on behalf of their unknown master. Indeed, an enemy is messing with the space time continuum and Jotaro is travelling through space and time to gather a Saint's Corpse
After their defeat, Jotaro uses Star Platinum's time stop to prevent Rohan from escaping and successfully restores him. Okuyasu escapes, despite Josuke's best efforts to give chase. The group later converges at Boyoyoing Cape, where Josuke glimpses a group of Harvest's units and attempts to gather the rest, remembering the death of their user Shigekiyo Yangu at the hands of Yoshikage Kira. One group escapes and returns to Shigechi, who confronts Josuke and Jotaro alongside Akira Otoishi. After being defeated, both enemies escape. Jotaro finally forces Joseph to explain his relation to a confused Josuke inside Mr.President. Rohan and Koichi join Jotaro, but Yukako Yamagishi attacks the two, only to be defeated and disappear like the other enemies. Jotaro is suddenly confronted by Enya the Hag, who sends a group of corpses under her control to attack the group. Though the puppeteered corpses are defeated, Enya extols "his" virtues and "his" power to "overwrite" his enemies before disappearing. With the coast clear, Jotaro finds the Corpse's left arm and entrusts Josuke with it. Joseph then emerges from the turtle, revealing a camera he found inside. Via Joseph's Hermit Purple, the group finally glimpses their true enemy: the photo reveals none other than DIO himself, despite his certain death at Jotaro's hands.
From that point on, Koichi travels with the party across space and time to help gather the pieces of the Saint's Corpse.
Unfortunately, DIO springs a trap and Koichi is among the 36 souls that DIO absorbs in order to attain even greater power. Fortunately, Jotaro manages to defeat DIO once and for all, and fate is altered for the better.
History pursues its course. In the year 1999, Koichi encounters an incredibly tall man in his home town of Morioh, accompanied by his young daughter. The man had decided to bring her along, supposedly because she refused to stop crying until he caved in. After the girl runs off to look at a turtle, the tall man asks Koichi about the location of a certain Josuke Higashikata...
Koichi is last seen during the ending credits where a photo shows him alongside Josuke, Okuyasu and Shigechi who has now survived, enjoying tea together despite Kira having changed his identity to that of Kosaku Kawajiri.