Ichimatsu Matsuno is the fourth-born son of the Matsuno family and one of the identical sextuplets. As an adult, he is distinguishable from his brothers by his half-droopy, half-closed eyes, his slightly ruffled and messier hair, a hunched posture, and his preferred casual attire of a purple hoodie, blue sweatpants with a white stripe, and purple sandals. He sometimes wears a white medical mask, which adds to his withdrawn appearance. In childhood, he was nearly indistinguishable from his siblings, though he was characterized as the most serious, strong-willed, and intelligent of the six, despite still struggling academically.

As an adult, Ichimatsu is defined by his personality as a cynical and asocial "lone wolf" who holds a very low opinion of himself. He is a quiet individual who speaks in a low mumble and prefers to keep his distance from others, largely due to a deep-seated lack of self-confidence and a fear of social rejection. This cold and threatening exterior is a defensive mechanism for a surprisingly sensitive and fragile interior. This vulnerability is visually represented in a gag where his heart is depicted as being made of glass with a "fragile" sticker on it. His motivations are not driven by grand ambitions; as a NEET (Not in Education, Employment, or Training), he spends most of his days idly, finding his only consistent comfort in the company of stray cats, for whom he has a well-known and deep affinity.

In the story, Ichimatsu's role is often that of the dark, pessimistic foil to his more energetic or delusional brothers. Despite his loner nature, he is frequently drawn into the family's collective schemes and misadventures. His most significant relationship is with his younger brother, Jyushimatsu. The two are often seen as a pair, with the hyperactive and cheerful Jyushimatsu being one of the few people Ichimatsu tolerates and even shows a protective, caring side towards. In contrast, he displays an open and intense animosity towards his brother Karamatsu, whom he derisively calls "Kusomatsu" (Shittymatsu). While the reasons are complex, it is suggested that this hatred stems from jealousy or an inability to understand Karamatsu's unwavering, if painful, self-confidence. His relationship with Choromatsu is marked by discomfort, as Ichimatsu feels intimidated by Choromatsu's attempts to live a "normal" and productive life.

Ichimatsu undergoes significant character development, particularly as explored in the Mr. Osomatsu movie. It is revealed that his current dark personality is a direct result of a traumatic experience in high school. During that time, he adopted a popular, outgoing persona to fit in and differentiate himself from his brothers, going by the nickname "Ichi". This facade eventually collapsed under the pressure of maintaining social relationships, leading to a breakdown that fundamentally shattered his self-esteem. This past failure cemented his belief that he is incapable of friendship and social success, explaining his adult insecurities and his retreat into a solitary world with only his cats for company. When confronted with his younger self, his reaction is so visceral that he experiences suicidal and violent impulses, highlighting the depth of his trauma.

Despite his human flaws, Ichimatsu possesses several notable, often quirky, abilities. He has a well-documented fear of syringes, which contrasts sharply with his usual demeanor. In several comedic gags, he physically transforms into a cat or grows cat ears, usually when his affinity for felines is at the forefront. He is also known for his sadistic and violent tendencies when provoked, taking the lead in torturing their rivals Iyami and Chibita on more than one occasion. Other skills include a talent for origami and, as demonstrated in a later season, possessing a driver's license.