- Main article: Nagisa Hazuki
With only three members on the swim team, forming a medley relay team was impossible, so that’s why Nagisa sought out Rei. At first Rei exclaimed “If people evolved to walk on land then it makes no sense for them to return back into the water!!!” but because of Nagisa’s persistent persuasion, he entered the Swim Club. Trying his hardest to catch up with the others along with Nagisa who follows with all his might, the two make an uneven combination, but only they can eventually calm the tides of the Swim Club.
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. Nagisa often falls asleep on Rei's shoulder on the train ride home. Rei easily gets manipulated by Nagisa for some reason.
- Main article: Haruka Nanase
Haruka and Rei have a good senpai-kohai relationship. When Haruka saw Rei practice pole-vaulting for the very first time, he was amazed by how amazing he looked doing so. When Nagisa told Haruka and Makoto that Rei didn't join the swim club, stating that Rei might hate water, Haruka gained a big dislike for Rei, saying that they don't need someone like that. When Rei finally met Haruka in person after running to school with Nagisa, he asked Haruka if he was Haru-chan-san Nagisa was talking about. Haruka cut him off telling him not to use both '-chan' and '-san' on his name. After Rei saw Haruka swim at the joint practice with Samezuka Academy, he realized that he wants to have the kind of freedom Haruka is. Haruka told him that it's not freedom that he has, but freestyle.
Rei looks up to Haruka immensely, and thinks that his swimming is beautiful.
- Main article: Makoto Tachibana
Makoto and Rei have a good senpai-kohai relationship. Makoto tends to act towards Rei as an older brother and often draws him away from Nagisa's constant hyperactive activities.
Makoto was ready to risk his life in order to save Rei from the storm.
- Main article: Gou Matsuoka
- Main article: Miho Amakata
- Main article: Rin Matsuoka
Rei and Rin's relationship at first is very hostile. Rin is clearly shown to be jealous of Rei (in season 1) because he does not think he's good enough for the Iwatobi team (Haruka, Makoto, Nagisa) that even though he doesn't want to admit, wishes he was still with. He is very bitter towards Rei, making statements such as, "you call that the butterfly?" Rei confronts Rin one day, asking him his true intentions for Haruka and the others. Rin is very shocked at this, for this is one of the first times he truly has to confront his feelings.
Rei then leaves, and later on Rin contacts Rei saying he wants to finish what they started, and he will answer his questions. They meet up before the big match, and Rin explains himself to Rei. When the Iwatobi guys overhear Rin saying he wants to quit swimming, Haruka becomes very depressed, stating, "I won't get to swim with Rin again?" Rei, knowing he must do something, tells them the truth about Rin's feelings and how he wants to swim with the Iwatobi guys again. When Rin swims in the relay with the others instead of him, Rei is a bit sad and jealous, but knows it was the right thing, and doesn't mind.
After this Rin and Rei's relationship improves, and they are no longer hostile towards each other. In episode 1 of season 2 Rin even makes a playful comment towards Rei, saying "Be careful, you don't want to get bitten" before they race in butterfly.
Rei also trusts Rin enough to go to him to ask him for help with the other strokes (he can't swim them). He wants to improve them to impress the Iwatobi guys.
- Main article: Goro Sasabe