Koichi is generally terrified of his strict, over-bearing mother. He does his best to keep her at bay while he lives his life, as he believes that she would disapprove of his lifestyle and force him to go back home to the countryside. Koichi's mother does not approve of her son's love for super-heroics, begrudgingly agrees to let him act as a vigilante when Koichi is forced to save Makoto Tsukauchi from a runaway bus. After some words from Knuckleduster, she finally accepts that she cannot hold him back forever, and all she can do is help him when he needs it.
Mrs. Haimawari admitted that in his infancy, Koichi was initially able of limited flight, but worrying for his safety, forced him to never use that part of his quirk until he completely forgets about it.
Koichi first encountered Pop☆Step when she was making the rounds as her idol persona, alerting her when he noticed the cops were approaching the area. The two meet again after Koichi gets beaten up by some thugs and he decided to roam the streets helping people. When Pop☆Step gets attacked by some thugs and is almost sexually assaulted, Koichi is initially unable to act in her defense, but after some encouragement from Knuckleduster, he takes a stand and saves her from her attackers. From there she, Koichi and Knuckleduster begin working together to stop minor crimes in their neighborhood while also investigating the sudden rise of Trigger drug usage.
Unbeknownst to Koichi, he and Kazuho share a history. Koichi had once saved Kazuho when she fell into a river while using her Jump quirk. He would give the drenched Kazuho his limited edition All Might hoodie and run off to his entrance exam for a high school that specialized in superheroics. Because he chose to save her, Koichi ended up missing the exam and left Tokyo to return to his hometown. Meanwhile, Kazuho would keep the hoodie for many years, and when she encountered Koichi again years later, she would struggle to find a way to bring up their chance encounter and return the hoodie.
It appears that Kazuho harbors feelings for Koichi, but is unable to express them, displaying tsundere-like behaviour. Koichi is completely oblivious to this fact.
The two vigilantes share a loose mentor-mentee relationship, with Knuckleduster helping Koichi with combat training and initially helping him use his quirk more effectively. It was Knuckleduster that suggested that Koichi change his hero name from Nice Guy to The Crawler and pushed him to take his role as a vigilante. Despite this, the two aren't especially close, with Koichi being generally terrified of Knuckleduster's predilection towards violence. However, unlike Kazuho, Koichi is able to acknowledge that although his methods are rough and seemingly excessive, Knuckleduster knows when to hold back when it comes to the force of his attacks, showing a respect for his skills as a vigilante.
Koichi first encounters Shota on his first patrol with Knuckleduster to look for Trigger users. The old vigilante assaulted Mario, starting a fight against Shota who confused him for a villain. That day there was not much interaction between Koichi and Shota.
They do not meet again in person until several months later, while he is trying to avoid the Next-Level Villain Octoid. Koichi meets Aizawa by chance, and both recognize each other from their previous encounter. At that time, Octoid was on a rampage, so Shota tries to stop him, but is wounded in the fight. When Octoid was about to finish him, Koichi intervenes to save his life. With the help of the Hotta Brothers, Shota and Koichi collaborate to defeat Octoid.
After that confrontation, every time Koichi meets Shota, he greets him friendly, although Aizawa prefers to ignore him.
Koichi would have a chance encounter with a jogging Ingenium while experimenting with his quirk early in the morning. To Koichi's surprise, instead of admonishing him for using his quirk in public, Tensei instead gives him tips on using his quirk more effectively, allowing Koichi to move faster without speeding out of control. Seeing the potential in the young student's quirk and his genuine desire to be a hero, Tensei offers Koichi work at his agency, handing him his business card. The two would run into each other again when Koichi and his fellow vigilantes help Ingenium and his crew catch a runaway villain who had been evading them. Ingenium would recognize Koichi but would decline to go after him and his vigilante crew. The following morning, upon running into Koichi again, the young vigilante would deny his involvement with the villain's capture. Seeing that Koichi was providing a valuable service as a vigilante untethered by the law, Tensei asks for his business card back, saying that Koichi had already found his place in the world of heroics.
The two would meet once more when Koichi, his mother, and Makoto Tsukauchi were trapped in a runaway bus possessed by a Trigger-inflicted alley cat. Although Ingenium and his hero crew would be able to rescue most of the passengers, Makoto would remain trapped on the bus when the cat was injected with more Trigger. Ingenium and Koichi would work together to rescue Makoto. Once again, Ingenium would make no move to arrest or admonish the vigilante.
At first, Koichi could not stand Captain Celebrity because of his loud, boisterous and obnoxious personality, acting only when the cameras were present. It did not help either that he signed his All Might hoodie without permission, or knowing his womanizer attitude, he had a date with Makoto. Captain Celebrity, on the other hand, did not take Koichi too seriously.
Over time, and due to Captain Celebrity's attitude improvement thanks to Makoto's control over him, both begin to have more friendly interactions, and Koichi begins to respect him as a person and a hero. Captain Celebrity saves Koichi's life several times
Makoto and Koichi would meet in their university while Koichi was struggling with his Heroics Sociology course. Although Koichi has a negative reputation in the university after a misunderstanding involving his apartment, Makoto remained interested in the freshman and decided to help him with his studies, with the ulterior motive to use him as help for her thesis in Vigilantism. Makoto initially had the correct suspicion that Koichi to be the Crawler, but makes the mistake of asking him if he was the Cruller, which does not set off her Polygraph quirk when he denies it.
He would later ask Makoto to act as his girlfriend when his mother comes to visit, although Koichi's mother would immediately see through this ruse and realize that the two were not a couple (seeing Makoto as being too good for her son). Makoto and Koichi's mother would immediately befriend each other and make plans to go sightseeing in Tokyo. When Koichi, Makoto and Koichi's mother get caught in a runaway bus, Makoto would remain the last one trapped on the bus when Ingenium rescues the passengers. Koichi, acting quickly climbs onto the bus and saves her, but ends up revealing his identity as the Crawler to Makoto in the process.
It is implied that Koichi is romantically attracted to Makoto, and when asked by Koichi's mother Makoto states that the two being an item isn't out of the question. Whether she was merely saying this to be polite, or if she's genuinely interested in the younger Koichi is uncertain, but she is nonetheless fond of Koichi, even if she only thinks of him as a friend.