Aiichirō Nitori is a supporting character who appears from the first season of the series onward. He is a student at Samezuka Academy, a rival school to Iwatobi High School, and becomes a dedicated member of its prestigious swimming club. Physically, Aiichirō has short gray hair with straight bangs and light blue eyes. A distinctive beauty mark is located diagonally under his right eye. During his time in high school, his height is recorded as 168 centimeters, later increasing to 171 centimeters.

Aiichirō’s background in swimming dates back to elementary school, where he competed in the same tournament as Rin Matsuoka, Haruka Nanase, Makoto Tachibana, and Nagisa Hazuki. His own team lost in the preliminaries, but he was deeply moved after watching Rin and his teammates compete in a relay. This experience left a lasting impression and is the origin of his deep admiration for Rin. This admiration is the central pillar of his personality and motivations. He looks up to Rin as a mentor and role model to the point of wanting to emulate him, even striving to specialize in two swimming styles, breaststroke and freestyle, just as Rin does.

Personality-wise, Aiichirō is friendly and earnest, though he can be a bit of a chatterbox who sometimes speaks without thinking. An example of this is when he comments that the Iwatobi team members have girly names, only to realize that Rin’s name also fits that description. Despite being on the receiving end of Rin’s sharp tongue or occasional annoyance, Aiichirō remains persistent and bubbly, never giving up on his efforts to support his senpai. He is a hardworking swimmer who is very aware of his own limitations, which can lead to moments of frustration and self-doubt, especially when he feels he is not improving or when he is unable to compete in his preferred events. This insecurity sometimes contrasts with his earnest desire to push himself to the extreme in training.

His role in the story is primarily as a supportive underclassman and a conversation partner for Rin. The narrative often uses Aiichirō to draw out Rin’s thoughts and feelings, as Rin can see parts of himself in the younger swimmer’s struggles. Throughout the series, he transitions from a somewhat insecure first-year to a more confident leader. A significant part of his development involves overcoming the disappointment of being outperformed by a talented underclassman, Momotaro Mikoshiba. With encouragement and specific technical guidance from another senior, Sosuke Yamazaki, Aiichirō works relentlessly to improve his breaststroke. His dedication pays off when he earns a spot on the Samezuka relay team, achieving his dream of competing alongside Rin. By the end of his third year, Rin appoints Aiichirō as the captain of the Samezuka Swimming Club, a testament to his growth and reliability.

In terms of key relationships, his bond with Rin Matsuoka is the most defining. He initially calls Rin by his surname but is eventually allowed to call him Rin-senpai, while Rin comes to address him casually as Ai. His relationship with Momotaro Mikoshiba starts with some disappointment when Momotaro becomes his new roommate, but they grow to become close friends. He also shares a significant relationship with Sosuke Yamazaki, who helps refine his swimming technique. It is also revealed that Mikhail Makarovich Nitori, Rin’s coach in Australia, is Aiichirō’s uncle. Outside of swimming, his hobbies include playing with dogs, and his favorite food is borscht.

As a swimmer, Aiichirō’s primary and most developed specialty is the breaststroke. While he also swims freestyle, his notable ability is his powerful and technically improved breaststroke, which he develops through intense practice. He is known for being a hard worker rather than a natural genius, and his endurance and dedication are his greatest strengths, allowing him to slowly but surely break his own personal records and compete at higher levels, including national tournaments.