Ichimatsu Matsuno is the fourth eldest of the six Matsuno sextuplets. He shares the same basic facial features and build as his brothers but is most easily identified by his perpetually half-closed, sleepy-looking eyes, his noticeably hunched posture, and his messier, more unkempt hair. His casual outfit of choice is a purple hoodie with loose-fitting sweatpants and sandals, a look that sets him apart from his siblings. His birthday is May 24th.

Personality-wise, Ichimatsu is known as the "lone wolf" of the family. He is a quiet, asocial, and often gloomy individual who tends to mutter to himself and generally avoids interacting with other people. This behavior stems from a deep lack of self-confidence and a difficulty in expressing his true feelings, not from genuine misanthropy. Although he projects a dark and cold exterior, this is largely a protective shell; in a notable visual gag, his heart is depicted as being made of glass with a "fragile" sticker on it, suggesting a high degree of sensitivity. He has a very low opinion of himself and often displays signs of social anxiety. When provoked or agitated, he can become sadistic and prone to violent outbursts, frequently targeting his brother Karamatsu in particular.

His primary motivation is not driven by external goals like getting a job or finding romance, unlike some of his brothers. Instead, he seeks a way to cope with his own insecurities and feelings of alienation. While he claims not to need friends, he finds solace and companionship in stray cats, for whom he has a well-known and powerful affection. He is often seen holding, feeding, or simply sitting with them, and cats are the one thing that can reliably draw out his gentler side.

In the story, Ichimatsu serves as the darkest and most cynical member of the sextuplet group. His personality frequently drives plots centered on his own psychological struggles, his relationship with his brothers, or his misanthropic gags. He is often paired with his younger brother, Jyushimatsu, forming a classic "odd couple" duo where Jyushimatsu's boundless, chaotic energy contrasts with and frequently overpowers Ichimatsu's gloomy lethargy. Despite his irritability, he shares a close bond with Jyushimatsu and is often seen begrudgingly following along with his antics. In contrast, his relationship with Karamatsu is defined by open disdain and verbal abuse, which the more earnest Karamatsu typically endures without much retaliation. He also feels intimidated by Choromatsu's attempts at being normal and productive, highlighting his own feelings of inadequacy.

A key piece of character development is revealed in the film, Mr. Osomatsu: The Movie. It shows that as a popular and outgoing teenager, Ichimatsu adopted a false, confident persona to fit in. The collapse of this mask under social pressure was a traumatic failure that directly shaped his shut-in, insecure adult personality. When confronted with his younger self, his reaction is so extreme that he attempts suicide and tries to murder the teenager, showing the depth of his self-hatred and shame.

As for notable abilities, Ichimatsu is shown to have the greatest physical endurance of his brothers, as seen when he was the only one able to nearly scale a tall building in a chase. He is also portrayed as more practical and thrifty than his siblings, often being the one with a small amount of money saved up. While he possesses a sharp and perceptive mind, he rarely applies it to anything productive. Despite his asocial nature, Ichimatsu does care for his brothers in his own way, and on rare occasions, he will take initiative to care for them when they are sick, revealing his hidden, more nurturing side beneath the surly exterior.