Tsumugu and Chisaki met at Mihama Junior High. At first, Chisaki disliked Tsumugu because he seemed to show feelings for Manaka. She didn't want this because Hikari loved Manaka and she wanted him to be happy. However, after Hikari, Manaka, Kaname, and the rest of Shioshishio hibernate without her, she begins to get closer to Tsumugu. This eventually led to him having feelings for her. After the hibernation, Hikari confronts Tsumugu about Manaka. But Tsumugu confesses that he loved Chisaki, which she heard. She dives into Shioshishio and he follows her and gains Ena. He then confesses his feelings for her and she accepts them.
Chisaki originally loved Hikari, but after realizing her love for Tsumugu, she discovered her feelings for HIkari was only family-like. They are childhood friends.
Manaka and Chisaki are best friends and were also rivals in love.
Kaname was in love with Chisaki, but she failed to notice his feelings as she was in love with Hikari. Even after she wasn't in love with Hikari, her relationship with Kaname stayed only as childhood friends.