Mimi stands immortal, her body forever frozen in teenage appearance by a Time Fruit absorbed centuries ago. She maintains a bright, flirtatious personality, often making playful or romantic advances toward Rin Asougi and expressing openness to sexual relationships with other women, including informants.

Her eternal devotion to Rin began centuries past when Rin saved her from an angel, forging a deep sense of indebtedness. Serving formally as Rin's assistant at Asougi Consulting, Mimi handles finances, information, and formidable computer hacking, though she meets her match in superior hackers like Katsuyuki Kamiyama.

After Rin's 2025 disappearance and presumed death, Mimi abandons detective work, becomes a Buddhist nun, and cares for their immortal dog, Genta, while remaining vigilant for Rin's return. Decades later, upon learning of Rin's reappearance as the amnesiac Tamaki Saito, Mimi investigates, confirms her identity, and orchestrates a rescue mission to Apos's castle to save Rin from permanent imprisonment and torture.

Mimi's immortality forces perpetual regeneration back to her transformed state, trapping her youthfully. Her feelings for Rin evolve from early romantic or lustful inclinations into profound, self-sacrificial devotion. Declaring she loves Rin more than life itself, she commits to an eternal bond, driving her actions throughout their story, especially the final rescue.