Osomatsu Matsuno is the eldest of the sextuplets at the center of the anime Osomatsu-kun and the character after whom the series is named. Born first among six identical brothers, he acts as the leader of the group, although individual stories do not always place him as the main protagonist, with that role occasionally shifting to supporting characters. In later adult-focused adaptations, he appears as a young man in his mid-twenties who lives at home without steady employment and wears a red hoodie as his distinguishing feature, since the brothers otherwise share a similar face, black hair, and average build. His name derives from the Japanese word osomatsu, which means poor, lousy, or ill-prepared, a detail that reflects the comic shortcomings attached to him.

His personality is defined by laziness and a pleasure-seeking outlook. He tends to prioritize immediate enjoyment over long-term goals and generally avoids responsibility, responsibility avoidance and the pursuit of fun being the main principles behind his actions. Although he holds the position of the eldest and is sometimes treated as the most mature of the six, his behavior often betrays a lack of genuine accountability, and he is known for deflecting difficult situations with jokes or escape. He can be dramatic and self-deprecating, playfully insisting that he is not all that great, and he has a tendency to act without a plan. He is outgoing in social settings, enjoys attention and interaction, and has a fondness for gambling, particularly pachinko and horse racing. His behavior is frequently described as that of someone who has carried an elementary school mentality into adulthood, and within the story he functions as a norm-breaker whose disorderly conduct undermines the expectations that come with being the eldest son.

His core motivations revolve around seeking fun, avoiding work, and preserving the comfortable status quo of living together with his brothers. He is attached to the shared household and resists the social pressures of adulthood and employment, and he tends to act to keep the group together even when doing so is not in anyone's best interest. He also shows a sentimental side in matters of romance, carrying a long-standing crush on Totoko, an idol-like figure known to the family since childhood, for whose attention he competes alongside his brothers.

His relationships with his five brothers, Karamatsu, Choromatsu, Ichimatsu, Jyushimatsu, and Todomatsu, are central to his role. He frequently teases and manipulates Choromatsu, whose more sensible tendencies make him an easy target, and he often drags Karamatsu into irresponsible schemes such as gambling. His dynamic with Ichimatsu is marked by a casual closeness without deep involvement, and his interactions with the others tend to swing between brotherly camaraderie and petty rivalry. Beyond the family, he and his brothers frequently clash with recurring characters such as Iyami and Chibita, whose schemes the sextuplets typically manage to thwart.

In terms of development, the character is largely static by design. In a later one-shot story set after the original series, he is depicted as an ordinary unmarried salaryman. In the modern sequel adaptations, storylines about job hunting and confronting the family's social stagnation offer occasional moments of self-awareness that hint at underlying sincerity, but the series generally resists sustained growth or reform, using his unwillingness to change as a source of comedy.

His notable abilities include being the best fighter among the sextuplets, with impressive hand-to-hand combat skills and occasional clever tactics in tight situations. However, these talents are undercut by his laziness, indecision, and tendency toward self-sabotage, so fighting ability usually serves to amplify comedic situations rather than solve problems. Like his brothers, he also holds a driver's license.