Chisaki Hiradaira is a main character from the anime set in the seaside village of Shioshishio, where she lives as one of the sea people. Initially fourteen years old, she is a childhood friend of Hikari Sakishima, Manaka Mukaido, and Kaname Isaki. She has long, wavy purple hair typically styled in a high side ponytail and the distinctive sea-blue eyes of her people. She is an only child with loving parents who are described as strict but supportive of her growing independence.

Chisaki is characterized by a serene, thoughtful, and mature demeanor, often acting as the most level-headed and responsible member of her friend group. Hikari even compares her role to that of a mother, as she is constantly looking out for Manaka and trying to keep the group together. Despite this outward calmness, she is quite shy, lacks self-confidence in matters of the heart, and has a deep-seated fear of change. She values the unchanging nature of her relationships above all else and becomes distressed when she perceives that new people or surface influences might alter the dynamic she holds dear.

Her central motivation for much of the story is her unrequited love for Hikari. This feeling began in childhood when he saved her and Manaka after they got lost, and the act of him holding her hand sparked her initial affection. However, she knows Hikari loves Manaka, and because she cherishes her friendship with both of them, she hides her true feelings, believing that revealing them would ruin the group's balance. This internal conflict is a primary driver of her actions, leading to moments of jealousy and self-criticism, especially when she feels she cannot be as honest or as helpful to Hikari as Manaka is.

Chisaki’s role in the story becomes uniquely transformative following the Ofunehiki festival. In a catastrophic turn of events, while her three childhood friends fall into a deep hibernation, Chisaki alone remains awake on the surface. This event is the catalyst for her entire character arc. Filled with guilt and grief over being the one left behind while her friends slept, she is taken in by the Kihara family and slowly builds a new life over the next five years. She cuts her hair shorter, matures physically into a young woman, and pursues a career in nursing, motivated in part by the illness of Tsumugu Kihara's grandfather, Isamu, who became her adopted guardian.

This time skip creates the central conflict of her later development. When Hikari, Manaka, and Kaname finally awaken, Chisaki is now nineteen years old, while they are physically and mentally still fourteen. Having been the one who most feared change, she is now the one who has changed the most, and she feels like an outsider among her dearest friends. She struggles with guilt for having lived a relatively happy life without them and clings to the memory of her old feelings for Hikari as a way to prove she is still the same person, even though those feelings have naturally evolved.

Her key relationships are complex and interconnected. Her friendship with Manaka is one of deep care and protectiveness, but it is also tinged with the pain of jealousy. With Hikari, her dynamic shifts from a secret admirer to a close friend and confidante, whom she eventually comes to love like a younger brother rather than a romantic interest. Kaname, who has long harbored feelings for her, confesses his love, which shocks her and adds another layer of complexity to her emotional state. Ultimately, her most significant relationship develops with Tsumugu Kihara. Initially resentful of him for being a surface dweller who attracts Manaka's attention, he becomes her closest confidant during the five years of solitude. They live together as a family, and she gradually falls in love with him, though she initially denies these feelings out of loyalty to her past self and her sleeping friends. Tsumugu, in turn, is in love with her, and with the help of Kaname, they eventually confront their true feelings and become a couple.

Notable abilities include her strength as a sea person, which allows her to breathe underwater. However, a significant aspect of her development is her choice to embrace life on the surface. She takes great pride in her heritage but ultimately gives up her ena, the membrane that allows underwater living, because she learns to love her life and the people on the surface more. She is also a capable and dedicated nurse, representing her nurturing personality channeled into a professional skill.