- Main article: Rin Matsuoka
Sosuke joined Sano SC in the third grade of elementary school when Rin was already a member and could swim butterfly. Rin forced Sosuke to join and learn swimming butterfly too and Sosuke would always be left saying "Ugh, just shut up". Since Rin was faster and wasn't saying anything wrong or untrue, Sosuke let him trying to teach him butterfly, even though it greatly pissed him off. Their relationship kept being like that for over a year up until Sosuke became a good match for Rin. Because Rin was the one who taught Sosuke how to swim, things like the timing of their breaths or the way they moved their arms were exactly alike and, as Sosuke once stated, he couldn't seem to shake it, it was just ingrained then. During the time he was at Sano SC, Sosuke and Rin's relation was having competed for being the representative, formerly they certainly had a relationship that could be called friends.
When Rin said that he wanted to go Australia and when he said that he wanted to swim at Iwatobi SC, just saying ‘I understand’ while he was half shocked, he didn’t try anything like halfheartedly keeping him back or pressing questions. Sosuke kept being someone who understood Rin well until the end.
In High☆Speed!, Rin stated that it’s better if he's not on the same team as Sosuke because they "understand each other too well". He says that Sosuke’s even more theoretical than he is and that they often quarreled. They settled on it that the faster way is always correct, but their competitive spirit wasn’t half-baked anymore. He said that he doesn't hate him. Rather, he still thinks of him as the person who understands him the most, but sometimes it becomes difficult to be together. When he ends up understanding Sosuke's feelings, he ends up not being able to say the things that he wants to. He could no longer seriously go up against Sosuke. because he was that one friend who got him way more than his other friends, and that made Rin feel uncomfortable. He said that that was not a friend, but like an alter-ego (you don’t like or hate your alter-ego).
Even though Sosuke and Rin didn't see each other in such a long time, they're still great friends. Sosuke likes teasing Rin, as seen when Rin asked him if he's going to stop swimming and has a special type of a handshake with him. Sosuke is very concerned about Rin and his future and dislikes everybody who stands in Rin's way to success, including Haruka, who, according to Sosuke, did "something Sosuke would never do".
- Main article: Aiichiro Nitori
Aiichiro greatly admires Sosuke, be it because he is a high class swimmer (being in the top ten butterfly swimmers), or because of his long relationship with Rin. At first, Sosuke seemed to dislike Aiichiro, giving him a cold look when he tried to approach him, but then he quickly smiled and ever since then, they're on good terms. While Aiichiro was training to gain a place in the regionals, Sosuke offered to train him. Alongside Rin and Momotarou, Sosuke and Aiichiro competed against Team Dolphin, losing at the end.
- Main article: Momotarou Mikoshiba
- Main article: Haruka Nanase
Sosuke initially does not like Haruka and thinks he gets in Rin's way. He confronts him in the second episode of Eternal Summer to tell him this. However as the season progresses Sosuke starts to understand Rin and Haruka's relationship better, and although he still doesn't accept Haruka he wants Haruka to do his best so Rin can do his best. In the OVA Sosuke and Haruka are shown finally making peace with each other and getting along.
- Main article: Makoto Tachibana
- Main article: Nagisa Hazuki
- Main article: Rei Ryugazaki
- Main article: Gou Matsuoka
- Main article: Miho Amakata