Aiichirō Nitori is a male character introduced as a third-year high school student at Samezuka Academy. He possesses light blue eyes, short gray hair styled with straight bangs, and a distinctive beauty mark diagonally beneath his right eye. His height is recorded as 168 cm during high school, later increasing to 171 cm. Born on January 4, his astrological sign is Capricorn. His connection to competitive swimming began in elementary school, where he participated in the same competition as Rin Matsuoka, Haruka Nanase, Makoto Tachibana, and Nagisa Hazuki. Witnessing Rin's relay victory profoundly moved him, cementing a deep admiration that shapes his character. This admiration leads him to emulate Rin, partly due to sharing a room at Samezuka, and fuels his rigorous dedication to mastering both breaststroke and freestyle. He maintains a persistently bubbly and earnest demeanor despite occasionally irritating Rin. Recognized at Samezuka as a hardworking swimmer, Aiichirō acknowledges his limitations yet pushes himself relentlessly. He experiences frustration and self-doubt when unable to compete in preferred events, especially when contrasting his situation with Rin's versatility. His emotional vulnerability surfaces when he misinterprets Rin's comment about "seeing his father" as a suicidal intention, reacting with genuine distress and tears. His commitment to team protocol is evident when he firmly reminds Rei Ryugazaki to change into proper swimwear during joint practice. Following Rin's graduation, Aiichirō ascends to captain of the Samezuka Swimming Club. He later enrolls at Shimogami University, continuing his swimming career. During this period, he participates in local festivals alongside Nagisa Hazuki, Rei Ryugazaki, Gou Matsuoka, Momotaro Mikoshiba, and Sosuke Yamazaki, indicating sustained social connections. Personal details include a preference for borscht and a hobby of playing with dogs. His Samezuka room is notably untidy, much to Rin's annoyance, containing items like an elementary school journal on Daphnia and a preserved umbilical cord in a grey box. Speculation about asthma arises from an inhaler-like item in his desk drawer and audible wheezing in supplemental media. The characters of his name translate to "love," "one," and "son" for his given name, and "similar" and "bird" for his surname.