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Ikuya's relationship to Haru is only seen in the Free! High Speed Starting Days movie. At first, Ikuya pays only slight attention to Haru by glancing at him in class. However, after watching Haru swim for the first time (tying with his older brother) Ikuya sees Haru as a role model. He starts imitating Haru from eating to swimming stretches (which Makoto notices). Later it is revealed that Ikuya imitates Haru in order to win back his brothers love (something he never lost in the first place) and make him pay attention to him more. In the end, Ikuya learns to appreciate Haru as a teammate -not just a role model. When Haru passes out from low blood sugar, Ikuya helps in cooking meals for him and spends the night at his house. Afterwards, Haru also accepts Ikuya as a teammate and agrees to swim with him and the others in the relay. Along with Makoto and Asahi and foursome win the relay where they all hug each other.
Ikuya and Asahi first meet in Class 1 during the Free! High Speed Starting Days movie. The two instantly clash when picking clubs. This is especially shown when Ikuya shows his disdain for school swimming clubs. Ikuya states that swimming teams in school are useless as the instructors don't teach you proper methods of swimming. Asahi disagrees stating that Ikuya was only afraid of jumping into something he had no idea about. In order to prove him wrong, Ikuya joins the swimming team along with Makoto, Haruka, and Asahi.
Upon entering the club, Ikuya is withdrawn and would rather swim alone than with others. Haru agrees on this idea. However, Asahi disagrees. Throughout the film the two continue to clash with Asahi desperately trying to be Ikuya's friend. Ikuya tries as well, showing concern for him when he can no longer swim freestyle. They often make fun of each other and are shown walking home together.
Sooner or later, Ikuya says he wants to quit and runs off crying. When Ikuya cries and tells the team that he was lonely and missed his older brother, Asahi is the first to lend his shirt in an effort to console him. Though is baffled when Ikuya fully bursts out in tears. When returning to the swim club after the incident, Asahi speaks up first saying they were late because he wanted to help Ikuya. Though Ikuya fully explains later. The two still have joyful bickering and Ikuya calls Asahi an idiot for his attitude. Yet they do care for one another and help each other when Haru passes out. In the end, Ikuya learns to enjoy or at least tolerate Asahi's company.
Makoto and Ikuya first meet as Haru and Makoto are being drafted onto the swim team. Sadly, Ikuya and Makoto don't have much of a relationship outside of the swim team due to being in different classes. However, they do value each others company and work well together (probably because Haru and Ikuya have a similar aura). Makoto is the second to notice Ikuya's imitation of Haru (after Haru of course). Yet dismisses it after asking Haru about it. When Ikuya finally tells the team about his loneliness and longing for his brother's company, Makoto consoles Ikuya stating that no older brother can hate his sibling. The two of them are probably face their feelings the most. Together with the rest of the team, they open up, help Haru when he was sick, and win the relay race.
Natsuya is Ikuya's dearest older brother of which at the start of the movie Ikuya has hated him for two years. When they were young, Ikuya follows his brother around often and looks to him as a best friend. A young Natsuya is seen encouraging Ikuya to swim more as he is getting better and better. However, when entering Middle School, Natsuya abandons Ikuya telling him to make new friends. While coming from a good place as an older brother, Ikuya takes this painfully and decides to withdraw himself from his brother and society. He becomes more silent, eats and sleeps less. He is also shown crying after being in an argument with his brother.
At first, whenever the two come in contact, the atmosphere becomes heavy and Ikuya becomes silent. However, Ikuya still looks up to his brother. When seeing Haru tie with his brother in a race, Ikuya starts to imitate Haru so his brother can concentrate on him. Ikuya is slightly jealous of Haru in this way. Ikuya often stares at Natsuya when he's not looking.
Despite Ikuya's hatred for him, Natsuya still shows concerns for his welfare and encourages him to say on the team. When helping Haru carry class materials, he asks him about Ikuya and how he feels about the team. Haru notices this and reiterates it to Ikuya while he cries to the team.
After Ikuya confesses his feelings to the team and to his brother, they appear to be on better speaking terms.
They are seen training together near the end of the film.