Kaname Isaki is one of the main characters from the group of four childhood friends who live in the sea village of Shioshishio. He is a fourteen-year-old boy who, along with his friends, transfers to a middle school on the surface after their underwater school closes. Among his peers, Kaname stands out for his calm, laid-back demeanor and his tendency to observe situations rather than react impulsively. He is intelligent and considers his words carefully, often acting as the voice of reason who can de-escalate conflicts, particularly those started by his more hot-headed best friend, Hikari. Despite his composed exterior, he possesses a subtle sarcastic wit and can make unexpectedly sharp observations that surprise those around him. This cool-headedness allows him to analyze the complex emotional dynamics within his friend group, to which he has long felt like an outsider looking in.

A core part of Kaname’s character is his deep, unrequited love for his childhood friend, Chisaki Hiradaira. Having liked her for many years, he has silently watched her feelings for Hikari, understanding that her attention was never on him. As the threat of a long hibernation looms, he tires of being a bystander and confesses his feelings to Chisaki, a moment that creates an enduring, if unreturned, connection between them. His motivations are largely driven by a desire to protect the relationships within his circle while secretly yearning for a place where he is someone’s first choice. During the Ofunehiki ceremony, after helping to rescue Tsumugu Kihara, a boy from the surface, Kaname is swept into the sea and enters a five-year hibernation.

The story’s second half begins with Kaname’s awakening, where he finds that the world has moved on without him. Chisaki has aged five years and grown close to Tsumugu, which forces Kaname to confront the painful reality that his place in her life has permanently changed. This period is a time of significant development for him, as he struggles with feelings of displacement and jealousy, realizing he is no longer even a bystander in Chisaki’s affections. His key relationships evolve considerably. His friendship with Hikari remains a strong, supportive pillar, with Hikari often relying on Kaname’s advice to navigate his own romantic troubles. His rivalry-like dynamic with Tsumugu softens as he accepts the inevitable connection between Tsumugu and Chisaki.

The most transformative relationship is with Sayu Hisanuma. Before the hibernation, Kaname was kind to the younger, troublemaking girl, helping her to confess her wrongdoings and calling her a good girl. He never noticed she had developed a crush on him. Five years later, Sayu has grown up while he remained frozen in time. She confronts him, revealing that she has always loved him and has been waiting for his return. This confession is a turning point; it makes him realize he was not entirely forgotten and that someone’s heart was set on him. Moved by her sincerity, he promises to stop seeing her as a child and to consider a future with her as a girl his own age. In terms of notable abilities, Kaname is a strong swimmer with good physical stamina, having placed well in school swimming competitions, even outperforming the more aggressive Hikari. Ultimately, his greatest ability is his emotional resilience and his capacity for self-reflection, allowing him to gracefully let go of a decade-long love and open himself to a new, hopeful beginning with Sayu.