Хірадайра Чісакі

A calm and compassionate 14-year-old middle school student from Shioshishio, a village located in the sea. Chisaki is the childhood friend of Manaka, Hikari, and Kaname, and together they attend Mihama Junior High School, which is located in the surface town of Oshiooshi.

She attempts to act mature in most situations, putting her friends' feelings before her own, but her temper can flare up whenever she gets especially frustrated - in particular towards Tsumugu, whose new presence she believes is spoiling the harmonious relationship between her friends.

Chisaki harbors an unrequited crush on Hikari, although he is unaware of her feelings. She also has a complex about her body​ and hates change to the point where it hinders her own emotional development.

Chisaki is the only one of her childhood friends left on the surface after the disaster at the Ofunehiki festival leaves Shioshishio in a state of forced hibernation, and she grows to be five years older than the three of them, feeling guilty for not only living peacefully away from her home in their absence, but for falling in love with Tsumugu in the process too. She aspires to become a nurse after Tsumugu's grandfather, who took Chisaki in after she was left homeless, collapses due to deteriorating health.