A female classmate who he somehow fished up in the beginning of the story (fomerly thought as a "fateful meeting" by Hikari). They interact quite a bit in the beginning but interact a tad less when the surface starts to have saltflake snow. He also ended up fishing her up again at the ending of episode 12.
After the time skip Tsumugu states (to Hikari) that he does not have feelings for Manaka.
A boy from the sea and his new classmate. Because of Hikari's onesided rivarly towards him, the two did not get along at first. Tsumugu tried to be friendly on his own quiet way, given his interest about the sea village and their culture. It took time for Hikari to accept him and after Tsumugu builds a pool for the sea-children to soak in, Hikari starts to consider Tsumugu a real friend and begins to call him by his first name. Nevertheless Hikari stills feels a bit jealous about Manaka's attraction towards him.
A girl from the sea who is his new classmate. While not interacting much at first, after being asked by her to be her "sea slug" and confessing to him how she is troubled by her feelings for Hikari, the two of them start to talk to each other more often. Tsumugu goes out of his way to give her advice, such as telling her that forcing herself to change or that neglecting her feelings towards Hikari is wrong, though she is not happy to be told the latter. He seems to trust her, speaking with her a bit about his mother, a subject he doesn't seem too comfortable about. In the 13th episode when he falls from their boat Chisaki immediatly dives into the ocean to save him. In episode 19 it's stated that he loves Chisaki when he was talking to Kaname.
Tsumugu's new classmate that came from the sea. The two do not interact very often, but Tsumugu does consider Kaname as a friend, a feeling that seems to be equal on both sides. Though Kaname gets really jealous when Chisaki is with him. On episode 13, when Tsumugu falls into the ocean Kaname helps Chisaki to save him.
After the time skip, Kaname acts somewhat harsh toward Tsumugu, but Tsumugu knows he only does so out of spite.