Misa is the immortal aide to the prominent underworld figure, the Bearslayer. Her ageless existence is intrinsically linked to the central myth of eternal life and the Mermaid's Scale treasure, making her a target for antagonists like the arms dealer Himuro, who kidnaps her. This act sparks the central conflict, compelling others to find both Misa and the treasure before its power is misused.

She shares a deep, possibly familial, bond with Maki, a 14-year-old seeking Lupin's mentorship, presented as Maki's search for her "sister figure." Furthermore, the secrets tied to Misa's treasure reach into the past of Lupin's lineage, implying her role connects to the legacy of Arsène Lupin the First.

Misa functions primarily within the narrative's core mystery, embodying the enigmatic allure and danger of its immortality myth. Her presence and circumstances directly challenge both protagonists and antagonists.