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.
During the three years that they have been working as the Naruhata Vigilantes the two have been supporting each other, both in the confrontations against villains, as well as in personal projects unrelated to vigilantism: Koichi always encourages Kazuho in his desire to be an Idol, and Kazuho really wants to Koichi to become a real, licensed Hero.
Koichi sees Kazuho as a friend and ally, completely oblivious to the fact that Kazuho harbors feelings for Koichi, but she is unable to express them, showing a tsundere behavior, which together with how dense Koichi usually is and his inability to read the room, causes several misunderstandings between the two. In fact, despite they have known each other for years, Koichi actually knows very little about her: where she lives, her family, her likes, her dislikes, etc.
All this begins to change after the disappearance of Kazuho, who was kidnapped and transformed into the villain Bee☆Pop by Number 6. All that situation makes Koichi rethink his relationship and his feelings towards Kazuho, and when he discovers that she is being controlled by a parasitic bee, Koichi goes out of his way to rescue her, even putting his own life at risk.
At first, despite saving him from some bullies, Koichi was terrified of Knuckleduster due to his quarrelsome temperament and his tendency to use violence to solve any problem. Therefore, when Knuckle duster offered to be his master and help him become a hero, Koichi rejected the offer, not wanting to have anything to do with him. However, after his confrontation with Mario, Koichi agrees to become his ward.
Since then, the two vigilantes shared a loose mentor-master 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.
Koichi respects Knuckleduster, admiring his determination, courage, and sense of justice when fighting villains. Despite this, Koichi is 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.
Like Kazuho, Koichi is completely unaware of Knuckleduster's past as the Pro Hero O'Clock, as well as his personal crusade against the Villain Factory, being surprised when he left without saying anything to anyone. Several years have passed since that, and Koichi has not seen him again. Sometimes he wonders where Knuckleduster is.
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 Shota 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 Shota prefers to ignore him.
Koichi casually met Tensei Iida and and they quickly became friends. He met him while practicing with his Quirk. 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. Considering that he had potential, Tensei invites Koichi to join his Hero Agency, and in this way he can become a Pro Hero over time. However, when he discovers that Koichi is the Watcher The Crawler, he kindly tells him that he cannot join his agency, much to Koichi's chagrin.
Despite knowing the truth about Koichi, Tensei never makes a move to arrest or admonish the vigilante neither reveal to anyone that he is The Crawler, and even considers him one more ally in risky situations. For his part, Koichi is grateful to him for covering up his secret identity, as well as admiring him for his chill and friendly personality, as well as being an exemplary hero.
Koichi met Captain Celebrity when he saved him from being hit by a truck, and despite being thankful, Koichi soon began to being unable to stand the American hero because of his loud, boisterous and obnoxious personality, as well as well for being in the hero business completely for fame and nothing else, to the point of not helping him until 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, for his part, didn't take Koichi too seriously at first. Although he recognized that Koichi had courage, Captain Celebrity did not hesitate to remind Koichi that he is not a Pro Hero, and his interventions simply complicated things.
Over time, and due to Captain Celebrity's attitude improves thanks to Makoto's control over him, both begin to have more friendly interactions. Koichi begins to respect him as a person and a hero, discovering that despite his initial arrogant attitude, Captain Celebrity is actually a good person. There comes a point where both of them risk their lives to save each other.
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 attracted to Makoto, feeling jealous every time he saw Makoto with Captain Celebrity before she became his manager. For her part, Makoto was fond of Koichi, even if she only considered him as a friend, but after the Tokyo incident, she did develop a small crush to the Vigilante. However, their relationship was no more than a friendly one, with Makoto helping him and Kazuho for several months, until she left for the U.S.A. to study.
The situation changes two years later, when Makoto raises his return to Japan after having success in the United States. During a visit to prepare for her return, Makoto finally confesses to Koichi her feelings for him and her desire to have a life together. Koichi is caught off guard by her proposal but rejoices. Even so, Makoto decides to give him some time to think about his decision since she also takes into account Koichi's relationship with Kazuho.
At first, Koichi was afraid of Soga and his friends Rapt and Moyuru, since the three of them used to bully him and humiliate him. Soga even beat him up when he thought Koichi was making fun of him. Things got worse when Soga and his friends assault Kazuho, and Koichi and Knuckleduster defeated them. In revenge, Soga acquired Trigger for himself and his friends, and they injected it, turning into instant villains and violently attacking Koichi, although they were defeated again and sent to the police.
Koichi met Soga again, and although he no longer had any intention of taking revenge for what happened, Koichi did not have a high opinion of Soga, whom he considered a thug and a villain. Their relationship begins to improve after Koichi risked his life to protect his from Stendhal. Although Soga does not recognize it, he is grateful for it, and even begins to show certain signs that he also cares for him. Likewise, Koichi also begins to interact with him in a more friendly way, without caring about his past attitude and their confrontations.
Koichi and Soga eventually become allies during Bee☆Pop's attacks. Soga rescued Koichi after he was defeated by Bee☆Pop, explaining the situation to him and helping him with his training as well as devising a plan to rescue Kazuho.
Being Soga's best friends, Koichi also suffered from Rapt and Moyuru mocks and humiliations, and they also tried to get revenge on him after their first defeat, being defeated again. After this, Koichi's subsequent interactions with the two of them are friendly and without any bad blood between them.
After he was rescued by Soga, Rapt and Moyuru took care of him until he recovered. They also helped him with his training to deal with Bee☆Pop, and participated in the operation to rescue and free Kazuho from the Queen Bee's influence.
Koichi met Vigilante Stendhal for the first time when he saved his life from the Instant Villain Akira Iwako. When the villain runs away, Koichi thanks him for saving him and asks him his name. Before leaving, Chizome reveals to Koichi that he is called Stendhal and is a fellow vigilante. This meeting made a great impression on Koichi, who starts gushing over Stendhal. He even has the idea of acquiring a katana like his and using it during his work as a vigilante, just like him.
But this changed when he discovered the truth of his savior, and that is that Stendhal ruthlessly killed all the villains he faced. Stendhal had been hired to assassinate Soga, and upon seeing that, Koichi immediately tried to save his former bully despite Stendhal's warnings to not to get in his way.
Stendhal praises Koichi's innate act of goodness but still considers him evil since those who side with evil have evil within themselves, and tries to kill him too until Knuckleduster arrives to save them.
Despite having interfered with their plans on several occasions, even having helped to make several of them fail. Koichi does not know anything about Number 6, in reality he does not even know its existence, although the villain knows him very well.
Number 6 despises Koichi on a professional level for his constant and inadvertent interferences, and on a personal level because his idol O'Clock, whom Koichi knows as Knuckleduster, rejected him in favor of Koichi. Therefore, for years Number 6 devised a plan to gain fame as the hero O'Clock II, in which he would also destroy Koichi at all levels, starting with kidnapping Kazuho and transforming her into the villain Bee☆Pop.
Koichi finally meets Number 6 for the first time after he rescued Kazuho. The villain, under his person as O'Clock II, tried to kill them both, but Kocihi manages to escape with Kazuho, making his plan fail in the worst possible way. Shortly after this, Number 6 meets Koichi again, and this time is when the young vigilante is truly aware of Number 6's existence and his true intentions, when the villain cryptically reveals his origins and swears to Koichi that he will kill him, before leaving to recover from his injuries.