Rei Ichidō leads the Kimengumi, a group of misfit students united by eccentric appearances and poor academics. Nicknamed "Mayunashi no Rei" (No-Eyebrows Rei), his family runs a toy store. He lives with his father and younger sister; his mother is deceased. His father openly favors the sister, viewing Rei as a disappointment due to his academic failures and immaturity, framing Rei as the irresponsible sibling against his capable sister.

Rei struggles academically, requiring multiple attempts to pass high school entrance exams and frequently failing subjects. Despite this, he possesses unexpected athletic prowess, often winning sports competitions against favored opponents despite his poor physical education reputation. His personality blends simple-mindedness, childlike innocence, and eccentricity. He embraces his class clown role, employing cartoonish reality manipulation like voluntary shapeshifting for comedy. These traits define him as an "Idiot Hero" – determined and big-hearted, yet unintelligent.

Rei founded the Kimengumi, uniting formerly isolated oddballs. Under his leadership, they learned self-acceptance and developed their signature humor. Though respected as their leader, his friends sometimes treat him as a scapegoat. A recurring element is his relationship with Yui Kawa, an academically gifted and popular girl drawn to his humor and eccentricity despite widespread confusion over her attraction. This dynamic persists throughout the series without significant status change.

Rei's personal growth remains static across all seasons, films, and spin-offs. He maintains his kindhearted nature, cloudcuckoolander tendencies, and resilient spirit despite frequent humiliation or failure. His home life includes interactions with his dog Lassie, reflecting his affinity for animals. Throughout all official storylines, Rei embodies the "Loony Friends Improve Your Personality" trope, having transformed from a lonely outcast into a self-accepting individual who values friendship above conventional success.