Aiichiro seems to be friendly and a slight chatterbox. Sometimes he speaks a little too much, like when he mentioned that all the Iwatobi swim team members have girly names, but is quickly reminded by Rin's glare that Rin also has a girly name.
While running with Rin, Rin says to Aiichiro that he didn't know there was going to be a joint practice with the Iwatobi swim club. Aiichiro tells Rin that the captain made the call, and mentions that the club's team members all have girly names. However, he immediately cuts himself off after Rin glares at him and is reminded that Rin also has a girly name, to whom he quickly apologizes to. When Rin asks him how does he know, Aiichiro tells him he was in the same swimming competition as them and says that the race was amazing to watch.
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.