Osomatsu is the eldest of the six identical Matsuno brothers and the title character of the film. In his twenties, he remains an unemployed, idle adult who lives at home with his family and passes his days in the company of his five younger brothers. The story places him at the center of a takoyaki party held at the Matsuno household, a gathering that grows into a legendary overnight stay joined by childhood friends and female acquaintances, giving Osomatsu and his brothers an unexpected and chaotic chance to prove themselves as adults.

His background is inseparable from that of his brothers. All six were born as identical sextuplets, and because Osomatsu was born first, he has always claimed the position of leader of the group. Even in adulthood he introduces himself as the eldest and behaves as the self-appointed spokesman of the family, although his authority is rarely taken seriously by anyone, including his brothers. He is lively, loud, and self-assured, with a fiery and pushy personality that leads him to plunge into situations without much thought. He carries himself like a troublemaker, yet his smooth and charming manner tends to make him well-liked. His laziness is as pronounced as his confidence, and he would rather spend his time joking and loafing than accepting genuine responsibility.

Osomatsu's motivations are simple and immediate. He seeks fun and stimulation, avoids boredom and work whenever possible, and craves recognition as the eldest son and leader. He also shares his brothers' desire for romantic attention, and the film's sleepover, which brings Totoko and the other female characters into the Matsuno home, represents a rare opportunity for him to chase that goal. In the story his role is that of the instigator and ringleader. As the takoyaki party gradually descends into greater and greater chaos, he is one of the main forces pushing the evening forward, whether by leading his brothers into bold plans or by making the situation worse through his own impulsiveness.

His relationships are defined by his bond with his five brothers, Karamatsu, Choromatsu, Ichimatsu, Jyushimatsu, and Todomatsu. The six of them constantly bicker, tease one another, and drag each other into trouble, but Osomatsu's connection to each brother is close, and beneath the surface of the chaos he acts as an older brother who ultimately looks out for the others. He also has a long shared history with childhood acquaintances such as Chibita and Totoko; the film brings these familiar faces together under one roof, and Osomatsu relates to them through the same blend of old familiarity and selfish scheming that marks his everyday life.

In terms of development, the film pulls the brothers out of their ordinary stagnant routine for a single night of heightened activity. Osomatsu is placed in situations that demand more from him than lounging around, and moments of cooperation, embarrassment, and self-awareness reveal a more sincere side beneath his bravado. He does not undergo a dramatic transformation, but the night tests his confidence and his sense of his own place within the family.

His most notable ability is physical. Among the six brothers, Osomatsu is the strongest fighter and is capable of holding his own in confrontations and brawls. This strength, however, is undercut by his habitual laziness and his tendency to reach for the easiest or most entertaining option in any situation. His real talent lies in his force of personality, as he can talk his way through problems and rally his brothers into action, even when the results of their actions turn out to be far from sensible.