Mimi Usami attends Otogi Academy as the childhood rival of Otohime Ryugu. Their antagonism originated in elementary school, where Mimi bullied Otohime by mockingly calling her "turtle" while displaying a deceptively sweet demeanor to others. This two-faced behavior continued into high school, marked by Mimi's open hostility and jealousy toward Otohime's physical transformation and popularity.

The rivalry climaxed during a school popularity contest where both competed. Otogi Bank's intervention caused both to lose—Mimi received zero votes while Otohime received only one, inadvertently revealing Tarou Urashima's genuine care. Throughout the contest, Mimi directed vulgar language and aggressive outbursts at Otohime.

Later, Mimi requested Otogi Bank assistance during a promotional discount. Unknowingly acting as bait, her request led Onigashima High thugs to target Alice Kiriki. Upon realizing she had lured Otogi Bank members into danger, Mimi intervened to rescue Alice. She then joined the Otogi Bank confrontation at Onigashima High's entrance. Initially hesitant to fight, she intervened decisively to save Otohime from harm upon seeing her endangered.

This incident prompted Mimi and Otohime to reconcile, apologize for their years of conflict, and establish a friendship. Mimi later appeared as a prospective bride in another storyline.

Physically, Mimi stands 142 cm tall with light pink hair. Her character draws thematic inspiration from the "Hare and Tortoise" fable, reflected in her rabbit-like aura during conflicts and her derogatory "turtle" nickname for Otohime. Her surname "Usami" incorporates the Japanese term for rabbit ears (Usagi no Mimi), reinforcing this association. The name "Mimi" translates to "beautiful."