- Main article: Rei Ryugazaki
At first, Nagisa was someone Rei could barely stand because of the way he talks, changes topics very often, is very pushy, super greedy, spills things, doesn't do his homework, messes with other people and drools when he sleeps. Even so, Rei's very thankful to him because Nagisa was the one who invited him into the swim club, and because of that he was able to meet Haruka, Makoto and Rin. He's the reason why Rei was able to notice the beauty of swimming and was the one who "changed his world". In one of the interviews with Rei, he admits that, even though Nagisa is pushy at times, he has an aura that Rei's attracted to and that he has him completely wrapped around his finger.
They both belong to a same class in school and take the same train line to go back home. Nagisa often goes to Rei's home and hangs with him most of his free time. They're often seen doing things together. Rei easily gets manipulated by Nagisa for some reason.
- Main article: Haruka Nanase
Haruka and Nagisa swam together as children, meaning they had a fairly close bond. Although Nagisa did not see them for some years, when they met again they reformed their friendship quickly. Nagisa admires Haruka's swimming and enrolls at his school to swim with him again, planning on opening up a swimming club. When he learns that Haruka stopped swimming competitively, he gets sad but instead jokingly asks about starting a Hot Springs Club.
Like all other members of the Iwatobi High School Swim Club, Nagisa calls Haruka "Haru-chan", adding girlish suffix "-chan" to his name.
Although Nagisa usually jokes around a lot, he is very attentive, and in one episode when Haruka is sick and sneezes, Nagisa states that someone must be thinking about him, and says it must be Rin. Haruka is very surprised at his comment, and Makoto says it's probably just because he's sick.
- Main article: Makoto Tachibana
Makoto and Nagisa swam together as children, meaning they had a fairly close bond. Although Nagisa did not see them for some years, when they met again they reformed their friendship quickly. When Nagisa ran away from home, Makoto was visibly worried about him and tries to get him to open up. Later, when Nagisa thought his mother was coming to force him back home, Makoto offered to allow Nagisa to hide behind him. Like all other members of the Iwatobi High School Swim Club, Nagisa calls Makoto "Mako-chan", adding girlish suffix "-chan" to his name.
- Main article: Gou Matsuoka
Nagisa seems to enjoy teasing Gou by calling her Gou instead of Kou, which is constantly upsetting her. Even though, the two seem to be on good terms and rarely fight (aside from him refusing to call her Kou). They often seen to have a same response, along with Rei.
Like all other characters somehow connected with Iwatobi Swimming Club, Nagisa calls Gou "Gou-chan".
- Main article: Miho Amakata
Like all other characters somehow connected with Iwatobi Swimming Club, Nagisa calls Miho "Ama-chan".
- Main article: Rin Matsuoka
Rin and Nagisa swam together as children, and had a fairly close bond. When Rin left for Australia he made no attempt to contact Nagisa or the other boys. When Rin returned, Nagisa was excited to see him. When Rin saw the Iwatobi swim team in the abandoned pool, however, he acted cold towards Nagisa, which upset him. Like all other characters somehow connected with Iwatobi Swimming Club, Nagisa calls Rin "Rin-chan", adding girlish suffix "-chan" to his name. Once Rin reconciled with the boys, him and Nagisa seem to have rekindled their old friendship and are very close.