Mana is a central character in the story, introduced as a fifteen-year-old girl who has lived her entire life in a remote village populated by women who are actually mermaids in human form. She was taken as an infant and raised in captivity, with her legs bound by shackles for fifteen years, to be kept as a sacrifice. The mermaids force-fed her the flesh of a mermaid, which granted her perfect immortality. Their plan was to eventually kill and eat her so that her immortal flesh would rejuvenate their aging bodies.

Her background has shaped a personality that initially appears cold and distant, almost to the point of ruthlessness. Having been waited on hand and foot her whole life, she also displays spoiled tendencies, though life on the road with her rescuer gradually curbs that behavior. Despite her sheltered upbringing, Mana is strong-willed, fiercely brave, and highly intelligent. She learns with remarkable speed, demonstrated when, immediately after being freed from her shackles, she forces herself to learn to walk within minutes. She is also quick to grasp new situations and adapt.

Her primary motivation at the start is survival—she is rescued by Yuta just as the mermaids are about to kill her. After escaping, her drive shifts to staying with Yuta and finding a place in the world. She becomes deeply devoted to him, seeing him as the only person who shares her condition and the only one who gives her life purpose. Her role in the story is that of Yuta’s immortal companion; the two travel together, encountering others affected by the curse of mermaid flesh, and she frequently supports and protects him in dangerous situations.

The most significant relationship in her life is with Yuta. It goes beyond romantic or sibling bonds; he is the first person to treat her as a human being rather than a tool, and she feels an intense loyalty and emotional attachment to him. Her character develops notably from a spoiled, sheltered girl into a capable and determined individual. She learns to fight for herself, to make decisions, and to face the harsh realities of their eternal journey.

As an immortal who successfully ingested mermaid flesh, Mana possesses the ability to live indefinitely without aging. She does not die from conventional injuries—she can survive wounds that would be fatal to ordinary humans, and she regenerates over time. Beyond her immortality, her most notable abilities are her quick thinking, her courage in the face of danger, and her growing physical competence as she learns to defend herself on the road.