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- Main article: Rin Matsuoka
Aiichiro said himself he knew Rin since elementary school. Initially, Aiichiro and Rin were not on the same team, however, once Rin joined the Samezuka Academy swim team, the two became teammates. Aiichiro thinks of Rin as a role model and is faithfully loyal to Rin, even after being yelled at or brushed aside by him throughout the series; he continues to cheer him on and support him. He cares dearly about Rin and earnestly respects him.
It is shown that Aiichiro's somewhat jealous of Haruka because Rin calls Haruka 'Haru', but keeps calling him 'Nitori'. However, when Rin called him 'Ai', he states that it makes him happy that he recognizes him as a fully-fledged athlete. According to the Drama CD, Aiichiro keeps a diary in which Rin gets mentioned on every page. He decides to buy Rin a diary as a Christmas present, as well.
- Main article: Seijuro Mikoshiba
Seijuro and Aiichiro are both on the Samezuka Academy swim team. They appear to be good friends, with a senpai-kohai relationship.
- Main article: Haruka Nanase
Aiichiro's relationship with Haruka is not deeply explored in the first season; however, early on in the series, Aiichiro is shown to be jealous of Haruka because Rin calls him "Haru". Nevertheless, it seems the two become good acquaintances/friends at the end of the first series.
- Main article: Nagisa Hazuki