Mitsuki Inokuma is the younger twin sister of Kōta Inokuma and younger sister of Yōko Inokuma. Initially a background presence in the first season, she gains slightly more prominence as a fourth-grade student in the second season. Physically, Mitsuki possesses long brown hair with distinctive forelocks sticking up on each side of her head and brown eyes. She typically wears long dresses paired with jackets, featuring ruffled accents, matching socks, and loafers.

Mitsuki shares a close yet complex bond with her siblings. Both she and Kōta habitually weave elaborate or unusual fabrications. Despite this tendency towards dishonesty, Mitsuki exhibits a quiet and introverted personality. She holds deep affection for her older sister Yōko, though this affection becomes notably strained if Yōko damages her treasured teddy bear. Mitsuki maintains an innocent demeanor and shows a slight dependence on Kōta, frequently appearing alongside him during family moments.

Living with her siblings, Mitsuki occasionally participates in events related to Yōko's school life. For instance, she and Kōta assist Aya Komichi in navigating romantic feelings toward Yōko, drawing on their observational insights into their sister. Her given name "Mitsuki" combines the Japanese characters for "beautiful" and "moon," while her surname "Inokuma" derives from "boar" and "bear."

Mitsuki appears across multiple franchise installments, including both anime seasons, the OVA *Kin-iro Mosaic: Pretty Days*, and the film *Kin-iro Mosaic: Thank You!!*. Throughout these appearances, her role remains consistently supportive, without significant individual development or standalone arcs. Her characteristic traits of quietness, occasional dishonesty, and sibling loyalty persist in all depictions.