Meimi Otonashi began as Naoto Jinno's shy classmate, secretly in love with him. Witnessing his confession to Rika Manabe ignited intense jealousy, leaving her emotionally vulnerable. This state allowed the demon Asmodeus to manipulate her, twisting her anguish to transform her into the demonic entity Misty Mei.

As Misty Mei, her personality became aggressively volatile and rebellious. She routinely ignored Asmodeus's commands unless they served her obsessive goal of reclaiming Naoto. Her volcanic temper erupted in explosive outbursts harming allies and enemies indiscriminately. Reverting to Meimi often brought Asmodeus's retaliation for her defiance.

Misty Mei relentlessly antagonized Rika Manabe (the angel Djibril), driven by jealousy and warped desires rather than demonic loyalty. She repeatedly attacked Rika using tentacle creatures. Her actions continued beyond the initial conflict, including a brief purification after Asmodeus's defeat. Rococo, Asmodeus's sister, later recaptured her using the "Eternal Worm" to forcibly separate Misty Mei from Meimi Otonashi once more.

Her involvement persisted in subsequent conflicts. She became connected to a mysterious photography student taking explicit images of students, coinciding with renewed demonic attacks targeting the angelic warriors, including the abduction of Hikari Jinno. Throughout these events, Misty Mei remained a persistent adversary, while her human identity, Meimi Otonashi, showed no sign of willingly collaborating with demons after her initial transformation.