Choromatsu Matsuno is one of six identical sextuplet brothers. As a child, he displayed notable speed and agility, useful in sports, but was characterized as quick to judge and quick to flee. He exhibited a short temper, selfish tendencies, and a propensity for violence, rudeness, and cruelty when acting independently, alongside cowardice under pressure. His childhood featured a strong partnership with the eldest brother, Osomatsu; they frequently collaborated on mischief and schemes. This relationship involved occasional power struggles, such as Osomatsu's temporary absence leading Choromatsu to abuse his authority, whipping his brothers and father until Osomatsu's return restored order. An extreme trauma occurred when Choromatsu inadvertently caused Osomatsu to become trapped in a cave for decades; the resulting guilt stunted Choromatsu's growth and left him mentally regressed, capable only of repeating his name. Certain childhood experiences hinted at later traits, including occasional laziness, unreliability, social awkwardness, and instances of misfortune.

During his teenage high school years, Choromatsu attempted to cultivate an image of responsibility and intelligence, wearing thick glasses and acting as a teacher's pet. This persona was largely performative, as he concealed erotic magazines within his textbooks and continued to earn poor grades due to a lack of genuine studying. His posture and movements became noticeably stiffer. An encounter with his adult self during a time-travel event involved a pep talk about responsibility, reinforcing the path toward his future anxieties about maturity and societal expectations.

In adulthood, Choromatsu positions himself as the most logical and concerned brother regarding their collective unemployment and NEET status. He frequently acts as the straight man, attempting to curb his brothers' antics and openly questioning Osomatsu's leadership while suggesting he himself should be in charge. This self-perception of maturity is contradicted by significant vulnerabilities. He exhibits intense idol otaku behavior, obsessively fawning over the cat-themed idol Nyaa Hashimoto and sharing the brothers' collective crush on Totoko Yowai. He struggles to conceal his emotions or thoughts effectively, often overthinking situations. Others, including Osomatsu and Totoko, have commented that his serious demeanor is not his true personality, revealing deep-seated insecurities, a tendency toward empty boasting, and underlying self-consciousness. His self-consciousness manifests physically as an enormous, unstable green orb dwarfing those of his brothers, symbolizing his crippling over-awareness and anxiety. Despite occasional efforts to change his life, he consistently reverts to NEET habits due to circumstances or a lack of genuine resolve. His conformity extends to authority figures; when separated from societal pressures, such as during an encounter with the Dayon tribe, he readily adopts new roles to fit in, only to face distress upon returning to reality. Attempts at employment often involve him conforming to workplace expectations at the expense of his usual personality.

Key relationships define much of his development. His childhood was dominated by his dynamic with Osomatsu, serving as a loyal right-hand man in numerous escapades, yet remaining emotionally dependent and prone to erratic behavior without him. In adulthood, this evolved into rivalry mixed with lingering, unspoken respect. His relationships with his other brothers are generally less defined, though he typically displays little respect for them unless pressured by authority. His romantic inclinations are overwhelmingly directed toward idols, particularly Nyaa, with whom he engages in obsessive fan behavior, admitting his interest stems from attraction rather than appreciation for their music. He also shares the brothers' collective, unrequited pursuit of Totoko.