Choromatsu Matsuno, the third-born Matsuno sextuplet, carries a name derived from an Edo-era term for fast-running, foolish children combined with the "chorochoro" onomatopoeia for rushing. Childhood depictions show him as swift to judge and flee, excelling in agility-based sports but possessing a short temper that prioritizes results over his brothers' welfare. He frequently allies with eldest brother Osomatsu, though their partnership fractures into power struggles when Osomatsu sabotages others or Choromatsu seeks dominance. During Osomatsu’s temporary adoption by a wealthy couple, Choromatsu violently seizes family control through whippings until Osomatsu’s return. His unreliability surfaces in critical moments, like dozing off during crises, and he endures humiliations such as elephant urination. He consistently disrespects his younger siblings.

As a teenager, Choromatsu adopts thick glasses and stiff posture to feign studiousness, though he hides erotic magazines in textbooks, neglects studies, and performs poorly academically. Adults find his forced teacher’s-pet persona obnoxious. This period establishes his later emphasis on responsibility.

In adulthood, he positions himself as the most logical and responsible brother, criticizing his siblings’ NEET lifestyle while aspiring to escape unemployment himself. He browses job magazines and condemns idleness, yet his responsible facade crumbles through inaction, unrealized plans, and complacency. His large ego fuels self-aggrandizement that irritates others. Though highly self-aware of his flaws, he fails to meaningfully address them.

His weaknesses include obsessive idol fascinations—particularly cat-themed Nyaa Hashimoto and Totoko Yowai—that trigger flustered reactions around women, concert attendance, and romantic otaku fantasies. This obsession contradicts his claimed maturity, as does his low resistance to female attention, exemplified by falling for transformed characters. He retains teenage interests in sexual content, suffering mockery as "Pajamatsu" or "Choropajerovski" after being caught masturbating.

Brotherly relationships remain complex: With Osomatsu, camaraderie coexists with rivalry; Osomatsu provokes him, yet Choromatsu stays emotionally tethered, evidenced by distress during brief separations. He occasionally supports Karamatsu—defending him from siblings or encouraging romance—though Karamatsu often abandons him. Choromatsu conforms to authority figures, shifting from childhood Osomatsu idolization to adult societal norms. He nearly weds into the Dayon tribe for acceptance before breaking down over his conformity and passively endures workplace teasing without assertiveness.

His development features episodes where inflated self-awareness manifests as a turbulent orb symbolizing repressed energy and maturity desires. Near-assimilation into the Dayon tribe or brief independent living underscore his struggle between societal conformity and NEET identity. Fleeting change attempts—like dyeing hair or job-seeking—always revert to baseline behavior, maintaining his sextuplet role.

He exhibits low alcohol tolerance, often first to drunkenness or hangover sickness. His adult appearance includes a triangular downturned mouth, smaller pupils, no cowlicks, and green clothing preferences—typically a hoodie over a white collared shirt.