Koichi Hirose/History

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 23, 1984 at 6:27 AM. He was born prematurely, only weighting 2325g and kept in intensive care for 7 days.

Koichi meets the Joestars
Koichi is a first-year student in class 1-B of Budogaoka Highschool. On the first day of school, he meets Jotaro Kujo and Josuke Higashikata, learning that they have extraordinary powers. Josuke being his first acquaintance in highschool, Koichi and him quickly become friends.

Some time later, Koichi is shot with the Bow and Arrow by Keicho Nijimura, who was searching for a Stand capable of killing his father. Koichi develops the egg-shaped Stand Echoes, whose ability is still unknown, and bears witness to Josuke's battle against Keicho as well as the theft of the Arrow by Red Hot Chili Pepper.

Fighting malevolent Stand Users
Koichi remains well-involved with the overall plot, his Stand even evolving multiple times as he fights a few battles on his own. He is a victim of the con artist Tamami Kobayashi, who threatens his family in exchange for money. In this battle, Koichi develops Echoes ACT1 and defeats Tamami, who later befriends him.

With the help of Tamami, Koichi investigates the Stand user Toshikazu Hazamada and tries to fight him alongside Josuke. Hazamada is ultimately defeated.

Yukako Yamagishi Falls in Love
Yukako, a fellow student in love with him, at first proved to be very difficult for him to deal with as she was very aggressive in her affections towards him. Koichi tried to look like a "bad guy" to keep her away but this plan backfires as she goes to the point of kidnapping him and threatening his life should he not appreciate her efforts to "change him into a better person". Koichi defeats her in battle after developing Echoes ACT2 but saves her life in the end.

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 then participates little in the battle against Akira, his weak long-ranged Echoes being used as a liaison between him on the port and Jotaro on the boat.

Koichi & Rohan Kishibe
Some time later, Koichi somewhat befriends Hazamada over their shared love of mangas. The pair goes to the house of famous mangaka Rohan Kishibe to ask for an autograph and are even invited inside. Rohan baits the students into looking at his art and they get turned into books by Rohan's Heaven's Door as a result. Rohan takes a keen interest in Koichi for his admirable personality and decides to brainwash him to serve as a muse. Thankfully, Josuke and Okuyasu intervene to beat Rohan and rein in his obsession.

A month later, Rohan and Koichi meet again. The two accidentally discover the Ghost Girl's Alley and explore it only to find Reimi Sugimoto, who explains that a killer has been lurking in town for decades. On the way out, Koichi is almost abducted by the malicious spirits of the alley but Rohan saves him. Koichi goes on to warn Josuke and Okuyasu about the killer.

Yukako Yamagishi Dreams of Cinderella
When Yukako searches for Aya Tsuji's help, Koichi is beginning to be attracted to Yukako. He even offers to become blind in case he fails to help Yukako return to her old appearance (changed by Cinderella). Later in the story, still lovestruck, she seemed to have learned from her mistakes in trying to force him and approached him more carefully, upon which Koichi warmed up to her and slowly, willingly, became her boyfriend.

Investigating Yoshikage Kira
After Shigechi is killed by the murderer Yoshikage Kira, Koichi becomes deeply involved with the investigation. He and Jotaro Kujo almost discover Kira's identity but must fight off Killer Queen's Sheer Heart Attack ability. During that battle, Jotaro is grievously wounded but Koichi actually develops Echoes ACT3 and stops Sheer Heart Attack. Kira comes soon after and defeats Koichi, who nonetheless remains defiant until Killer Queen impales him on his fist. Koichi almost dies but is saved by Jotaro and healed by Josuke. The group fails to capture Kira, who takes a new identity because of Aya Tsuji's Cinderella.

Later, Koichi, along with Josuke, Okuyasu and Jotaro search Kira's house, but the group is attacked by Kira's father, Yoshihiro. Yoshihiro is captured but tricks Okuyasu into releasing him and he flees with another Arrow.

Cornering Yoshikage Kira
Since Yoshikage has a new identity, Rohan and Jotaro take it upon themselves to investigate all the commuters in town, letting Yoshihiro Kira the chance to make several Stand Users and sic them at the heroes.

During Josuke's encounter with Highway Star, Koichi helps locate Yuya Fungami by tracking his Stand.

Koichi is kidnapped by Terunosuke Miyamoto, who traps him in a piece a paper with his Stand, Enigma. His life is put into danger when Terunosuke throws Koichi in front of a car, but he ends up being saved by Yuya.

Koichi also tries to save Rohan from Cheap Trick but almost rips out Rohan's back in the process.

Crazy Diamond is Unbreakable
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.

(The information below derives from the TV Anime. As such, it may or may not be considered .)

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.

The Book: 4th Another Day
The information below derives from a Light Novel not written by Araki. As such, it may or may not be considered.

After the events of Diamond is Unbreakable, Koichi and Rohan find a cat (named Trinita) covered in blood and the two try to find his master. Koichi soon discovers that Hanae Orikasa, the cat's master, 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 at the 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.

Gold Experience
Jotaro sends Koichi to Naples, Italy to locate Haruno Shiobana in order to get a skin sample for the Speedwagon Foundation to test, having asked Rohan to write 'Italian language skills' into him with Heaven's Door to make the ordeal easier. Shortly after arriving, Koichi notices Giorno performing in the street and then subtly bribing the policemen watching. Giorno offers to give him a ride, but Koichi refuses until noticing the long line of people waiting for a taxi.

Unfortunately, the offer was a trick and Giorno starts to drive off with Koichi's luggage before he can get in the car. Using Echoes ACT3, Koichi stops the car but Giorno simply smiles and flees the scene after transforming the luggage it into a frog.

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.

Gang Enrollment
Sometime later, when Giorno is undergoing the test to join Passione, Koichi locates his apartment and searches for his missing luggage. 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 leaves the apartment building only to stumble upon Giorno and a dead janitor. At first, he believes Giorno is responsible but soon learns that it was Black Sabbath's doing.

He ends up helping Giorno to defeat the Stand, noticing in the process that it possesses part of the Bow and Arrow. Koichi then accesses a payphone to inform Jotaro, but Giorno stops him and Koichi decides to remain quiet in deference to Giorno's virtuousness and behavioral similarity to those of the Joestar bloodline in the manga.

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.