Born in 1965, Mana was raised captive by a village of immortal mermaids who bound her legs and plotted to sacrifice her. Forced to consume mermaid flesh at fifteen in 1980, she gained immortality just before her intended ritual killing. Yuta, a five-hundred-year-old immortal, intervened at that moment, rescuing her and forging their partnership.

Mana instantly taught herself to walk after years of restraint, showcasing fierce bravery and decisive action in peril. Sheltered from the world, she lacked knowledge of animals or household tasks but absorbed new information rapidly. She formed unwavering loyalty to Yuta, joining his quest for mortality while developing naive, unclear romantic feelings.

Immortality granted Mana rapid healing from wounds like facial scratches. She fought alongside Yuta using knives against threats, including Lost Souls—monsters born from failed mermaid-flesh consumption. She repeatedly prioritized his safety, sucking poison from his injuries or shielding him physically.

During encounters with the eight-hundred-year-old Masato and his surrogate mother Misa, Mana kicked Misa away to protect Masato and battled a Lost Soul that had been his nanny, Yukie. These conflicts deepened her understanding of immortality’s psychological burdens.

Her bond with Yuta centered on mutual dependence and eternal commitment, with Mana vowing to stay with him forever. Her sacrificial past shaped her views on mermaid-flesh exploitation. Though her lineage remains ambiguous, some speculate ties to land-walking mermaids like Isago.