Misa emerges as a pivotal character in "Mermaid's Scar." A war widow bereft of her husband in World War II, she later lost her young son during a Tokyo bombing. Gravely injured and facing death in the ruins, she met a young boy, Masato. He offered her mermaid flesh, bestowing extended life and healing her wounds. Seeking a mother figure, Masato led Misa to assume that role for roughly eighty years.

Their bond centered on profound emotional dependence and control. Masato, immortal yet childlike in form, enforced this dynamic, compelling Misa to serve as his eternal caregiver. Decades strained this arrangement. Eventually, Misa's regenerative powers faltered, triggering her physical decline. As she weakened, Masato pursued replacements, planning to shift his dependency onto other women such as Yukie, the household nanny.

Misa's desperation mounted with her deteriorating condition. She hunted for more mermaid flesh to reclaim her fading immortality, driven by dread of death and bitterness toward Masato's manipulations. This pursuit erupted into violence, including an attempt to stab Masato with scissors thwarted by the immortal Yuta. Later, during an assault by Yukie—transformed into a monstrous Lost Soul after consuming mermaid flesh—Misa suffered fatal injuries. She perished in this attack, concluding her prolonged and tragic existence.

Originally unnamed in the manga and identified solely as "Masato's mother," the character received the name "Misa" in the OVA adaptation, which subsequent media adopted.