Hikari is a handsome young man with tousled brown hair swept to the right and striking deep blue eyes, an eye color shared by those born in the sea, reflecting the ocean’s hue. Prior to his hibernation, he is typically seen wearing the blue and white uniform of Nami Junior High, the underwater school he attended before it was shut down.

After awakening from his five-year slumber, Hikari retains a youthful appearance and is seen wearing his Mihama Middle School uniform. Outside of school, he often wears a casual blue hoodie paired with shorts, maintaining a laid-back and active look that suits his energetic nature.

Hikari initially exhibits a strong bias against people from the surface, perceiving them as those who abandoned the Sea God’s blessings for selfish reasons. He holds deep pride in his identity as a sea dweller and is fiercely protective of his origins, especially when it comes to those close to him, such as Manaka Mukaido.

His early personality is marked by impulsiveness, stubbornness, and a short temper. Hikari often acts before thinking, driven by emotion and pride. Despite these flaws, he is loyal, dedicated, and deeply caring. He often puts others’ needs before his own and is highly protective sometimes excessively so particularly toward Manaka. He dislikes feeling powerless and will push himself to act even in impossible circumstances.

As the story progresses, Hikari undergoes significant personal growth. He becomes more understanding and empathetic, recognizing that surface dwellers are not inherently different. Through his experiences and interactions, he matures into a more balanced and open-minded individual, striving for unity between the sea and the surface.

As a child, Hikari experienced a great loss with the death of his mother. Overwhelmed by grief, he cried for her to wake up, all while being consoled by his older sister, Akari Sakishima. From then on, their father took on the responsibility of raising both siblings, leading to a period of change in their family life.

Hikari attends Nami Junior High School beneath the sea alongside his close friends Chisaki Hiradaira and Kaname Isaki. One morning, while waiting for Manaka Mukaido to join them, he becomes concerned when she fails to arrive. He later discovers that she had been accidentally caught in the fishing net of a surface trawler.

During their introduction at Mihama Middle School on the surface, Hikari quickly becomes irritated by the discriminatory behavior of the surface students, who mock him and his friends for being from the sea. He retaliates with sharp words and urges Manaka to do the same, but is stopped by Chisaki. Later, during a physical education class, Manaka races Tsumugu Kihara whose grandfather was responsible for fishing her up earlier that day. In the middle of the event, Hikari stumbles and accidentally knocks both himself and Tsumugu to the ground.

That evening, Hikari speaks with Akari about the recent vandalism against her store. Upon returning to Shioshishio, he is confronted by members of the men’s group, who express growing frustration toward the surface dwellers. Troubled by the tension, Hikari visits Lord Uroko’s shrine with Manaka. There, he witnesses Uroko approach Manaka in a way that frightens her, resulting in her running off in tears. Uroko then places a curse on her.

The next morning, Hikari and his friends visit Manaka’s home, only to find that she refuses to leave. When Hikari speaks to her privately, he learns that Uroko’s curse has transformed her kneecap into a small fish head. Though frustrated, he quickly shifts to reassuring her. To help her return to school, he devises a way to hide the transformation, convincing her to go.

However, things worsen when Manaka runs away from school. Hikari, along with Chisaki and Kaname, begins searching for her but they are forced to retreat due to the drying of their skin on land. Undeterred, Hikari resumes the search later and finds Manaka with Tsumugu. Overcome with jealousy and frustration, he physically attacks Tsumugu before being stopped by Manaka. Though hurt by her apparent closeness to Tsumugu, Hikari escorts her home, emotionally conflicted about the growing distance between them.

Manaka Mukaido
Manaka is Hikari’s childhood friend and the person he has loved for most of his life. He is fiercely protective of her and often goes out of his way to comfort and support her. Though jealous of her interactions with Tsumugu Kihara early on, Hikari chooses to respect her feelings and silently supports her, believing her happiness is more important than his own.

After being pressured by Kaname to admit his feelings, Hikari confesses to Manaka, which leaves her overwhelmed and confused. She claims she does not understand love and runs away. Hikari eventually finds her, and the two reconcile, with Manaka saying she will reveal something important after the Ofunehiki ceremony. During that event, however, both fall into hibernation alongside Kaname.

Following the time skip, Hikari dedicates himself to finding and waking Manaka, eventually succeeding with the help of Kaname and Miuna. Once she awakens, however, she is unable to recall her feelings of love. Hikari suspects she once loved Tsumugu, not him, and while heartbroken, he prioritizes helping her recover what she lost. When the second Ofunehiki is arranged in an effort to restore Manaka’s emotions, Hikari encourages her and promises she will remember.

Ultimately, it is revealed that Manaka was in love with Hikari all along but suppressed her feelings to avoid hurting Chisaki. In the final scene, as they walk along the beach, Hikari tells Manaka he already knows what she wanted to say five years ago—signifying their mutual understanding and love.

Chisaki Hiradaira
Chisaki is one of Hikari’s closest childhood friends. She secretly harbors feelings for him, but Hikari remains unaware, as his heart belongs to Manaka. When he confesses to Manaka, Chisaki is hurt and nearly confesses her own feelings but ultimately refrains, fearing their friendship would change. After the time skip, she tries to force herself to continue loving Hikari to preserve the past. However, upon realizing that Hikari’s heart never wavered from Manaka and after hearing that Tsumugu loves her, she lets go of her feelings for Hikari and moves forward.

Kaname Isaki
Kaname is Hikari’s longtime friend and trusted confidant. The two have been close since childhood, often seen together alongside Manaka and Chisaki. Kaname frequently provides Hikari with a grounded perspective, and despite personal romantic frustrations, he remains a reliable friend throughout the series.

Tsumugu Kihara
Initially, Hikari disliked Tsumugu, believing him to be the object of Manaka’s affections. His jealousy and distrust were fueled by his prejudice toward surface dwellers. Over time, however, he comes to respect Tsumugu’s character and even encourages him to return Manaka’s feelings, not knowing that Tsumugu actually loves Chisaki. When Tsumugu finally admits this to Hikari, their relationship becomes more amicable, with Hikari accepting his sincerity.

Miuna Shiodome
Miuna is Hikari’s step-niece who develops romantic feelings for him. Although she hides these feelings out of guilt and the familial bond they share, her love for him causes internal conflict. Miuna plays a key role in the second Ofunehiki, volunteering to become the sacrifice to awaken Manaka’s emotions. When Hikari learns of her feelings, he is deeply regretful for having unknowingly caused her pain. Though it is never shown explicitly, it is implied that Hikari gently rejects Miuna’s feelings, and she finds the strength to move on, believing new emotions will come with time.