During World War II, Misa lost her husband and son in a Tokyo bombing raid. Severely injured and resigned to die in the rubble, she encountered a young-appearing boy who offered her a piece of meat. Unaware it was mermaid flesh, consuming it granted her immortality and healed her injuries. She named the boy Masato, viewing him as a replacement for her lost son, and they lived together as mother and child for years.

Initially harmonious, their relationship deteriorated as Misa grew increasingly distressed by Masato's unchanging appearance. After decades without aging, her frustration culminated in an attempt to strangle him. His subsequent revival revealed his shared immortality, shattering their dynamic. Unable to endure the eternal burden of unchanging motherhood, Misa began trying to escape him.

She eventually married a wealthy older man seeking normalcy, but a boating accident killed her husband and left Misa severely burned. Declared dead at the scene, she revived the next day, her burns healing completely due to her immortality. This incident drew Masato back into her life. Trapped once more, Misa resorted to violent attempts to kill Masato, including stabbing him, hoping to sever their connection and seize his mermaid flesh. These efforts failed as Masato always recovered.

Over centuries, Masato manipulated Misa into an endless maternal role, isolating her from mortal relationships. As her body deteriorated and lost its regenerative abilities, scars from Masato's attacks persisted. Fearing replacement after Masato targeted their housekeeper Yukie with mermaid flesh, Misa confronted him during Yukie's transformation into a Lost Soul. In the ensuing struggle, the monstrous Yukie fatally wounded Misa, ending her immortal existence.