This article lacks images. You can help out Free! Wiki by adding more .jpg, .png and .gif files that could illustrate this article.
- Main article: Gou Matsuoka
Seijuro has a noticeable crush on Gou after their first meeting. He is always eager for opportunities to speak to her, shown by his enthusiastic acceptance of an idea of a joint swim practice with the Iwatobi swim team. Nevertheless, Gou is somewhat oblivious to his advances and attempts, as she always denies using her so-called "sex appeal" to hook the Iwatobi Swim Club up with Samezuka Academy for training. His attempts to go on a date with her are often thwarted by over-protective Rin. He usually calls her 'Gou-kun', adding boyish suffix "-kun" to her name, much to her dismay.
- Main article: Rin Matsuoka
Initially, Seijuro and Rin did not know each other at the beginning of the series despite attending the same academy because Rin did not join the swim team at first. However, once Rin joined the Samezuka Academy swim team, the two became teammates and, so to speak, friends.
Despite being Rin's senior and captain, Rin does not like Seijuro at all, often addressing him simply as "Mikoshiba" when he's not around. However, the moment Seijuro appears, Rin addresses him politely as "Captain". in does not see Seijurou to be very responsible and finds that he is a bit too whimsical. However, he still respects him as his captain and fellow teammate. Rin does not tolerate Seijuro's attempts to go on a date with his sister and often stands in his way, being an over-protective brother he is. He also sometimes gets "an overwhelming urge to punch Captain to the ground" when Seijuro and Gou talk.
- Main article: Aiichiro Nitori
Seijuro and Aiichiro are both on the Samezuka Academy swim team.