Yuta, a 500-year-old immortal, acquired his condition after consuming mermaid flesh as a fisherman in the late 15th century. Originally living an ordinary existence, his inability to age gradually severed ties with his wife and forced him to confront his endless lifespan. Centuries of loss, isolation, and the lingering agony of rapidly healed injuries cultivated a profound weariness toward immortality, fueling his resolve to end it.

His journey spans Japan’s shifting landscapes, intersecting with others touched by mermaid flesh—monstrous “Lost Souls” twisted by its power and opportunists exploiting its gifts. Among them, he liberates Mana, a young woman groomed by mermaids for sacrificial rituals. Their bond, forged through shared immortality and mutual vulnerability, drives Yuta to shield her from a world unprepared for her innocence.

Decades of witnessing humanity’s darkness and mermaid-fueled tragedies have tempered Yuta’s kindness with pragmatic detachment. He resists entangling others in his quest yet intervenes to dismantle threats posed by fellow immortals, such as Masato, an 800-year-old manipulator preying on adoptive families, or Towa, a disfigured immortal pursuing vengeance through grotesque experiments.

Yuta’s abilities—rapid cellular regeneration, resurrection from intact remains, and combat expertise sharpened through survival—enable him to endure violent clashes, though resurrection exacts a toll through prolonged recovery. Despite his resilience, he clings to the hope of aging naturally, dismissing suicide as an escape.

Notable milestones include a tentative alliance with Rin, a Toba Island pirate leader offering fleeting refuge, and encounters with mermaids like Isago, whose insights expose underwater hierarchies. Traveling with Mana, he navigates moral gray areas: fleeting human connections amid eternal fatigue, and the harrowing costs of immortality etched into others’ lives.