He is an ordinary boy who befriends the malicious B.T. and tags along with whatever adventure the latter has.
Koichi makes a return in the spin-off one-shot Cool Shock Old B.T., which is set sixty years after the original series.
Koichi is a young boy of lean constitution. He has a lean face, a small pointy nose, oval-shaped eyes and tear-shaped eyebrows. He has dark wavy hair, cut into a regular haircut.
Cool Shock Old B.T.
In the Cool Shock Old B.T. one-shot set 60 years after the original series, Koichi has become an old man. He is rather short and stocky. His hair has whitened with age and he's kept the same haircut from his childhood, but has now a short beard that leaves a large patch unshaven around his mouth and is split between the nose and the lips. He wears a suit with a tie to which he adds a pullover. His jacket is a plain jacket with vertical stripes with clear and slightly darker stripes.
Koichi is a rather passive and meek boy, who's often frightened by what happens around him. That said, he's always put into strange and dangerous situation when he tags along with B.T. so he's often put into stressful situations. Koichi is a rather gentle soul and has a bit more courage than the average kid. For instance, he once tried to dissuade two bullies from beating B.T., who was grateful for the gesture and became his friend afterwards.
The first chapter narrates Koichi's meeting with B.T. and his grandmother. At school, Koichi meets with the two and is asked the direction to the principal's office to which Koichi obliges. B.T. playfully plays a trick on Koichi, pretending to spit many marbles out of his mouth when in reality he only spit one and had the rest of them in his sleeves.
The Summer Camp Incident
Koichi presents a disturbing incident that happened the last summer. In a summer camp, the present children were bullied by two upperclassmen, Kuroyama and Akagawa. However, B.T. was the only one immune to their taunting and was even defiant so the bullies decided to beat him up. For trying to help B.T., Koichi too was beaten.
Afterward, the children were shown playing by the lake, with Kuroyama putting a dragonfly in Akagawa's swimming pants for kicks. Koichi surprised B.T. with a sedated Japanese hornet in his hands. B.T. put the hornet in Kuroyama's jacket, also finding a knife in his clothes. However, it was Akagawa who was stung by the hornet which he found in his pocket instead, confusing Koichi. Later that night, Kuroyama was found stabbed in the guts with a knife. At first, Koichi believed that B.T. was the culprit but the boy explained that Akagawa did it and that he would get away with it since the camp's staff would silence the incident. Indeed, B.T. pushed Akagawa to stab Kuroyama by telling the bully that he saw his "friend" put something in his clothes earlier, referring to the dragonfly but letting Akagawa assume it was the hornet.
The Prank Corpse Incident
One day, Koichi surprises B.T. and an upperclassman named Ninomori plotting against another student named Date. Showing off his ventriloquy skills and how to fake a cardiac arrest by squeezing a ping-pong ball in his armpit, B.T. plots to swindle $1000 out of Date. It is revealed that B.T. is jealous of Date because he was courting a girl named Aiko whom B.T. had a crush on. For his part, Ninomori couldn't stand Date being better at kendo.
Koichi accompanies the two and witnessed Ninomori confront Date for a fake revenge. Ninomori notably breaks and stomps a memento from Date's grandfather, angering the man and causing him to make a serious strike. Ninomori falls and hits his head on a rock. Date is convinced that Ninomori is dead and B.T. barges in to blackmail Date, seemingly making him run to get the money. In the meantime, B.T. and Koichi discover that Ninomori really looks dead. Worse, Date comes back with a crowd and accuses the boys of having murdered Ninomori. Nonetheless, B.T. turns the situation around by using Date's memento and making it look like his grandfather is accusing Date from the otherworld. Ninimori actually comes to his senses, having unwittingly fooled everyone because his concussion from a sparring match made him fall unconscious.
Nonetheless, B.T. gets his wish as Aiko now looks down on Date for falsely accusing boys of murder.
The Two Old Guys Incident
Koichi and B.T. go fishing in the countryside, but their day is ruined when two strange men confront them. Those men are two thugs disguised as soldiers who kill animals for the kicks and take the kids prisoner. B.T. adopts a defiant attitude, amusing the leader who decides to make a bet. The soldier ties B.T.'s foot to the rear of a parked car nearby and asks B.T. to choose whether it will go left or right at the next intersection. If B.T. guesses right, a knife on the way will be available for the kid to free himself or else B.T. will be dragged across a harsh rocky road. B.T. chooses the left and the car does turn left; far from being lucky, B.T. has only analyzed the car's license plate and guessed its town of origin to know its most probable trajectory.
B.T. breaks his watch to get shards of glass and cut his bounds. The two men go at him, but B.T. uses a large array of tricks to keep them off him until the soldier grabs him and throws him into a truck's way. B.T. is almost run over, but mysteriously makes the thug run away by making him believe he's an ostrich. B.T. then performs a disappearance act to confuse the leader and throw a rock at his head, taking him out. It is explained that B.T. put a string around the soldier's tooth and tied it to the passing truck as he was flying, forcing him to run. B.T. also threw a rubber band at the leader's eye, blinding him for a moment and using a stick in his hands to throw himself up into the branches. As compensation for his watch, B.T. robs the leader of his money.
The Dinosaur Fossil Thief Incident
One day, B.T. and Koichi infiltrate a museum at night. It is revealed that Koichi has paid a visit to B.T.'s home, meeting an unknown woman on the way. More importantly, B.T. learns about and plans to steal the skull of a spinosaurus whose skeleton is being exposed at the museum. B.T. and Koichi enter the museum during the day and break into a storage room. A guard named Saiko catches Koichi, but B.T. distract him, allowing the kids to hide in a crate. In the process of stealing the skull, the boys trigger an alarm and Saiko comes to catch them.
B.T. sprays Saiko with a fire extinguisher to blind him, allowing the boys to run away. On their way to the exit, the boys cross path with the woman from earlier, but their interrogations are cut short when the woman goes away without explaining herself. B.T. and Koichi try to take an elevator but several guards come out of it. B.T. and Koichi nonetheless manage to enter the elevator without being caught, but are intercepted by Saiko. As a final ploy, B.T. bluffs Saiko into believing the stuff from the fire extinguisher and a hairspray will chemically react to create acid, and the guard is intimidated into letting the boys go.
The next day, B.T. confronts his grandmother about last night and she reveals that she manipulated B.T.'s fascination with fossils to goad him into stealing the skull. The ensuing chaos would then let her own agent, the woman, go into a safe room and steal the jewels stored there. B.T. can only acknowledge his grandmother's mastery, although he later tells Koichi that he did snatch a few jewels from the deposit point of the treasure by using Saiko's keys.
The Eerie Freckled Boy Incident
One day, Koichi's family is about to go out. Incidentally, B.T. discovers Koichi's well-trained dog. Just as it exits the garage, the family's car bumps into something and the family sees that they've hurt a freckled boy, later revealed to be named Manabu. The boy is fine but Koichi's parents feel guilty and invite him inside their home, hoping to not let the police know about the incident. Manabu quickly overstays his welcome, eating directly in the fridge and "borrowing" Koichi's clothes but Koichi's family do not know what they ought to do.
The next day, Manabu takes a liking to Koichi's dog and tries to make a bet. B.T. accepts on Koichi's behalf and the two boys each place a candy near an anthill to see whichever candy the ants will go to first. Amazingly, Manabu's candy is picked by the ant and he wins the dog. It is explained that last night, Manabu had placed insect repellent near the anthill and made it so B.T. would put his candy here. Next, Manabu's family arrives but instead of picking up their son, they pressure Koichi's family into letting them stay. The boy eventually privately gloats to Koichi and B.T. that his family has already intimidated the father into servitude and is planning to ruin him. B.T. decides to save Koichi's family.
This evening, B.T. serves a plate of sashimi to Manabu's family, only to reveal that it was blowfish sashimi. The whole family except the boy collapses, angering him and pushing him to pursue B.T. After the family is taken by an ambulance, Manabu sees the dog and foolishly follows him. Manabu eventually sees B.T. and throws a knife at him, but he's then bumped by a truck. Thus, the whole family of intruders has been pushed out of Koichi's home. B.T. reveals that he hasn't fed blowfish to them, but instead slipped cyanamide in it. The cyanamide prevents the digestion of alcohol, causing stomach pains and explaining why everyone except for Manabu have been affected. The series ends here.
The story begins with B.T. and Koichi walking together when an upperclassman suddenly appears. B.T. and the senior play a game (which they seem to have been playing for some time) in which B.T. tosses a coin in the air and catches it, and the senior tries to guess in which hand the coin is. However, the senior loses again and must give 100 yens to B.T.. As he angrily throws the coin at the ground, he demands a rematch but B.T. refuses. When the senior leaves, B.T. cheekily shows to Koichi that he had been hiding the coin behind his fingers to trick the senior. Then, an upperclasswoman gives the coin to B.T.. It is Fuyuko Nakagawa, a girl B.T. has a crush on. Seeing B.T. frozen still because of his shyness, Koichi teases his friend about Fuyuko.
Later on, B.T. and Koichi see Fuyuko being arrested by the police and learn that she's accused of having murdered a reporter. At the police station, a fat policeman interrogates Fuyuko, who confesses to the murder. Supposedly, the reporter had invited Fuyuko in his house because he had found her wallet but then had tried to assault her. She defended herself with a pair of scissors and accidentally stabbed the reporter in the back before fainting from the shock. A child and a pair of policemen stumbled upon the scene and Fuyuko was quickly arrested for murder, even if it was in self defense. Upon learning of this story through a policeman, B.T. decides to investigate the house and prove Fuyuko's innocence.
That night, B.T. and Koichi break into the reporter's house and stumble upon a secret envelope full of incriminating photos. The next morning, B.T. calls the fat policeman and accuses him of the murder. In truth, the reporter was merely hurt since he was able to rip his rug and it was the fat policeman (the only one who was present, alone and conscious when the reporter died) who finished him off. Moreover, B.T. shows a photo of the fat policeman working with the yakuza and concludes that the policeman must have had killed the reporter because he was being blackmailed. The policeman draws his gun to kill B.T. but the latter throws a rubber band at the policeman's eyes, blinding him for a second before hiding somewhere. Koichi and a second policeman reveal themselves and arrest the fat murderer. It is revealed that B.T. had hidden underneath a table and used two mirrors to cover his hiding spot.
Fuyuko is declared innocent and freed while the real murderer is imprisoned. However, Koichi has in his possession a photo of Fuyuko stealing a wristwatch. Koichi anxiously realizes that Fuyuko was also being blackmailed for her crimes. Worse, perhaps she actually intended to kill the reporter and made up the assault story to cover up her crime.
Cool Shock Old B.T.
At some point, Koichi gets married and has a son. When he is 35, he writes a novel based on the events of Cool Shock B.T. and asks Rohan Kishibe to draw the cover for him. When Koichi is around 59 years old, his son moves to the city.
By the time Koichi is 72, he lives alone as his wife has passed away for unknown reasons. One day when he is at home, he is greeted by two employees from "The Outlaw Guys" company. They flatter Koichi for being a popular author and claim that their company will be able to settle his retirement expenses. They go into detail about various different banking accounts and money-making methods such as mutual funds, foreign exchange accounts, cryptocurrency investment, and the like. Koichi laughs at them and says he isn't the type of old man to fall for something like that. He mentions that he has also already paid off his mortgage. However, the scammers make Koichi anxious by saying his house is violating the building standards act and it's possible he'll be arrested. They offer to sell him a single-family villa with a garden attached located in the suburbs of Morioh for 70 million yen. Koichi thinks that's a crazy amount of money but they convince him to sell his current house to pay it off so that Koichi's grandkids would be happy to visit him in the future.
Later, the scammers take Koichi on a tour to see the villa. Koichi eventually realizes it's a scam when the employees go into hiding and take his house's contract with them, along with stealing his life savings, pension, and late wife's inheritance, leaving him with a 70 million yen debt. Koichi loses his house and is forced to move out, causing his son and his son's family to be so dumbfounded that they stop contacting him. While Koichi sadly leaves from his home, B.T. arrives and offers to help him move. Koichi tells B.T. the whole story while they head to the villa. However, once they arrive, they find that the house is not the same as what Koichi saw in the tour, and is instead a run-down building. B.T. reveals that they tricked him by swapping the villa with a different one during the tour Koichi went on beforehand. Using a technique called structure relocation, they lifted the entire house with a jack and carried it elsewhere. B.T. examines the tracks on the ground and finds that they lead to Boyoyoing Cape which has a lot of conveniently empty houses. Once the tour was over, they moved the houses back to their original locations.
B.T. asks Koichi if he contacted the police, but Koichi said they didn't want to handle it since it was a legitimate contract and the scammers went into hiding. However, B.T. claims that's good for them since now they can get payback themselves. B.T. and Koichi create a ploy to lure the scammers to them. B.T. had his wife send furniture from London and then they uploaded a video saying that Koichi found rare diamonds in the furniture of an old house he bought (these were actually retrieved from a museum when they were kids). The scammers eventually see the video after it gets a lot of views and head to meet Koichi again. B.T. and Koichi get dressed up using B.T.'s stage clothing. Koichi dresses like a king, wearing lots of jewelry and the Red Stone of Aja. He pretends to thank the two scammers while they're baffled as to how he suddenly became so rich. B.T. dresses as a butler and brings them tea but as soon as they drink it, the male scammer collapses. B.T. then brings out a blade and intimidates the female scammer, claiming that he poisoned them with tetrodotoxin from blowfish. He says that if they want the antidote, she'll have to tell him where their hideout is and reveal the identity of their boss. Meanwhile, Koichi narrates that B.T. actually just put Koichi's own sleeping pills in the tea and not poison.
Out of fear for her boss, the employee refuses to say anything but B.T. manages to hack their car and find where their hideout is from the GPS navigation history. They head to the hideout with B.T.'s car and crash through the building. The boss reveals himself to be Manabu, claiming that he knew they'd try to hit him with a car again since they did with an ambulance 60 years ago. Manabu reveals that he already sold Koichi's house for seven million yen. He offers B.T. to make a bet, showing that he has a pouch full of coins. If he loses, he'll buy the house back since he still has the contract, and if he wins then he'll take B.T.'s vintage car. B.T. accepts and they decide that the rules will be to toss the coins onto tables and guess whether the difference between heads and tails will be an odd or an even number. Manabu goes with an even number so B.T. chooses odd. Koichi is the dealer and tosses the coins as the game begins. Koichi notices that B.T. was palming a coin that he pickpocketed. If he placed an extra coin on the table, B.T. could ensure the difference would be an odd number. However, Manabu also had a coin hidden in his palm. After both of them placed an extra coin on the table, Koichi also secretly placed a coin as well, guaranteeing that it would be an odd number. Dumbfounded at his loss, Manabu unsheathes his cane to reveal a sword and charges toward B.T. As he gets closer, Manabu is hit by the autonomous car that B.T. hacked and his sword goes flying, landing beside B.T. and puncturing his vintage car's tire. They use the car to send Manabu and his slaves to the police hospital. Meanwhile, B.T. opens Manabu's safe and remarks that he's glad Koichi caught on to his idea without any signals.
B.T. finds the contract of Koichi's old house in the safe but it'll take some time for him to get it back. He allows Koichi to stay over at his house for the time being, insisting that his grandmother would be happy to see him. However, since B.T.'s tire was punctured by the sword, they'll have to walk all the way there. B.T. steals a wallet he found in the safe, as he and Koichi prepare to go on the long trek back home. Koichi narrates that despite still being a wicked boy at his old age, B.T. is still his best friend.