Mimi Katsura is an elementary school student attending the same school as Illyasviel von Einzbern, Suzuka, Nanaki, and Tatsuko. She later becomes classmates and friends with Miyu Edelfelt and Chloe von Einzbern. While her past is largely unknown, she has a younger brother who worries about her and maintains the status of an honor student. She possesses no special abilities or magical capabilities.

Initially sweet, kind, and highly considerate of others, Mimi's personality shifts significantly after witnessing a misunderstanding involving Illya, Chloe, and Miyu. This incident, combined with being introduced to doujinshi—particularly boys' love (BL) content—by Suzuka and Suzuka's older sister, transforms her into a hardcore fujoshi. Her obsession leads to highly delusional behavior, causing her friends to view her as a lost cause. Her fujoshi tendencies dominate her daily interactions and creative pursuits.

A notable comedic trait is her weak presence, often resulting in others forgetting or overlooking her. Following her exposure to BL, she develops a specific talent for writing scripts for BL-themed doujinshi, channeling her fantasies into creative work.

Mimi's background includes a brief cameo in *Fate/hollow ataraxia* as one of the children fishing alongside Gilgamesh, predating her role in *Fate/kaleid liner PRISMA☆ILLYA*. She appears across multiple seasons of the anime adaptation, including *Fate/kaleid liner Prisma Illya*, *2wei!*, *2wei Herz!*, *3rei!!*, and the OVA *Prisma Phantasm*, as well as in the manga series. Her role remains peripheral, primarily serving as a source of comedic relief through her exaggerated fujoshi antics and situational forgettability.