- Main article: Ikuya Kirishima
Natsuya and
Ikuya Kirishima have a close relationship when they are younger. Natsuya constantly encourages Ikuya as he is learning to swim, assuring him that he is capable whenever Ikuya doubts himself, which is often. As the brothers develop their physical skills, Natsuya begins to understand that a competitive swimmer must also have mental fortitude in order to survive the pressures of competition and he sets out to develop independence in his brother, but does so without considering Ikuya's age and emotional fragility. Ikuya, confused and lonely, quits the
Bandou Swimming Club and withdraws into himself until forced into joining the
Iwatobi Junior High School Swim Club where he meets fellow first-years
Haruka Nanase,
Makoto Tachibana and
Asahi Shiina. Ikuya begins to feel better about himself until the team breaks up over the summer and Natsuya leaves Japan to attend school in the US for the next two years. Ikuya follows, but the brothers continue to have interpersonal issues until Ikuya enters university. There, he is able to confront his own weaknesses, make amends with others and reconcile with Natsuya, as seen in
Free!-Dive to the Future-.
- Main article: Nao Serizawa
Natsuya considers
Nao Serizawa to be his best friend. They are both competitive swimmers, though Nao is sidelined due to an eye injury and must wait until his surgery is completely healed before doing so again. Until then, he acts as the manager of the Iwatobi Junior High School Swim Club working with the first-year members. Nao is an insightful and understanding person who balances out the headstrong Natsuya when dealing with team dynamics, as well as the team captain himself. This is evident after Natsuya leaves Japan to compete as a free-lance swimmer on the international level. Though financially successful, Natsuya develops some unhealthy habits and ways of thinking which Nao, without lecturing, enables him to see, thus redirecting him towards a more productive path.
