Osomatsu Matsuno, the eldest of the Matsuno sextuplets, was a fiery and self-confident child who often asserted himself as the leader of his brothers. A natural troublemaker, he led his siblings into schemes that ranged from simple mischief to bullying those weaker than him. Despite his hooligan tendencies, his smooth attitude made him well-liked. He stood as the strongest fighter among his brothers and displayed pronounced gluttony and greed, especially for food and money, though he rarely managed to save any. His personality blended unreliability and willingness to betray his siblings for personal gain with occasional guilt and vulnerability. He hated green peppers, had cavities, and felt embarrassed by bedwetting incidents.

As a teenager, his personality stayed largely consistent into adulthood. He behaved crudely and without shame, engaging in acts like flipping girls' skirts in public. During this phase, he feigned embarrassment over his past but ultimately reaffirmed his self-assumed leadership role.

In adulthood, Osomatsu lives as a NEET with the mentality of an elementary school sixth-grader, described in his profile as a "Miraculous Idiot." He retains his bull-headed and self-proclaimed leadership traits but has grown more laid-back and foolish. He lacks motivation unless pursuing instantly gratifying goals, often sabotaging his brothers for amusement or personal benefit. His primary interests involve gambling, such as horse racing and pachinko, frequently funded by his brothers' savings. He exhibits lewd behavior and makes inappropriate remarks without shame, though he occasionally reveals a softer side to his siblings' struggles.

His adult appearance includes an upturned nose and a long, hanging mouth, with red-tinted highlights in his hair to distinguish him from his brothers. His image color is red, often reflected in his clothing, though he sometimes wears a formal blue suit. He is typically shown grinning widely and, in the first season, often rubs his finger beneath his nose as a character tic.

In the movie "Osomatsu-san: The Movie," Osomatsu and his brothers attend a high school reunion where their NEET status is exposed, leading to humiliation. Transported into a "world of memories" based on their past, they relive high school to address unresolved regrets. In this world, Osomatsu's younger self remains a shameless money-grubber and lecher. The experience forces the brothers to confront past insecurities and the impact of peer criticism on their diverging personalities. Osomatsu navigates these past events and participates in resolving the central regret, ultimately strengthening the brothers' bonds.

His relationships with his brothers are complex, mixing rivalry, dependency, and underlying care. He often voices frustration about being a sextuplet and wishes to be an only child, yet still feels a sense of responsibility toward his siblings. His interactions with characters like Chibita and Iyami range from rivalry to occasional cooperation. Despite his abrasive behavior, he is sometimes moved by his brothers' plights and attempts to help, though his efforts can backfire.

Across various media, Osomatsu evolves from a mischievous child to a NEET adult, maintaining consistent traits of self-proclaimed leadership, gambling addiction, and a blend of selfishness and sporadic loyalty. His development includes confronting past regrets and reinforcing his role within the family dynamic.