Rei Ichidō leads the eccentric Kimengumi as their protagonist. His family runs a toy store, and he lives with his father and younger sister after his mother's passing. Academically, he struggles profoundly, failing his high school entrance exam twice before passing and consistently ranking poorly in every subject – a trait common among his group.

His distinct lack of eyebrows earns him the nickname "Mayunashi no Rei" (No-Eyebrows Rei). Despite these academic shortcomings and a general reputation for underachievement in physical education, Rei displays surprising athletic prowess, often winning sports competitions against favored opponents.

Rei embodies a blend of foolishness and kindness, described as an "Idiot Hero" and "Kindhearted Simpleton." He proudly embraces his cloudcuckoolander tendencies while maintaining a fundamentally good nature. This combination attracts Yui Kawa, the intelligent and popular girl in his class, puzzling others due to Rei's status as a social misfit.

At home, Rei is the disappointing older sibling. His sister Kiri possesses greater intelligence and maturity, leading to parental favoritism from their father. Rei sometimes faces scolding under parental hypocrisy, as his father shares similar irrational traits. He finds companionship with his dog Lassie, a fellow cloudcuckoolander.

As the Kimengumi's leader, Rei anchors their dynamic of true camaraderie among social outcasts. Though occasionally treated as a scapegoat by members, their bond remains strong. He frequently employs toon physics and voluntary shapeshifting for comedic effect, embodying the group's embrace of absurdity. His core traits of simple-mindedness and good intentions remain consistent throughout his experiences.