Ichimatsu Matsuno stands as the fourth son among the Matsuno sextuplets. His name uses the kanji for "one" but alludes to the "ichimatsu" check pattern, a connection he reportedly dislikes when misspelled. He shares his brothers' physical resemblance but is marked by perpetually drooping eyes, slightly ruffled hair, and a constant slouch. His typical attire is purple casual wear: a hoodie and sweatpants featuring a white stripe, often worn with sandals at home, switching to the family's standard blazer and trousers for formal events.

Childhood revealed a serious, strong-willed nature, earning him the label "#1 serious" brother despite marginal intelligence compared to his siblings. Practicality defined him, often saving money, and he demonstrated superior endurance in physical challenges. While capable of kindness towards younger children, he could also display violence. He frequently teamed with Jyushimatsu for mischief.

A significant teenage crisis unfolded between ages 16-18. He adopted a popular, outgoing persona nicknamed "Ichi," masking deep insecurities. This strained facade shattered under social pressure, leaving profound shame in adulthood. A pivotal confrontation occurred when his adult self faced this past, attempting suicide and then trying to kill his younger self before restraint. This failure solidified his troubled adult identity.

As an adult, he embodies a "pitch-black" loner persona, avoiding human interaction due to crippling self-doubt and fear of rejection or disappointing others. He finds solace in cats, frequently feeding strays or bringing them home, exhibiting cat-like mannerisms such as partial transformation where ears appear when curious or startled. His brothers misinterpret his solitude as an inability to form friendships, though he finds comfort in their presence. Metaphorically, his heart is depicted as fragile glass labeled "fragile," signifying deep sensitivity beneath his aloof and threatening exterior.

His relationships with his brothers vary. He shares a close, enduring bond with Jyushimatsu, willingly enduring discomfort like acting as a counterweight on a baseball bat to support him. Conversely, he feels intimidated by Choromatsu's pursuit of normality and job-seeking, finding no common interests despite their shared middle-child status. He harbors overt hostility toward Karamatsu, even celebrating his misfortunes.

Trauma from his failed high-school persona manifests in extreme reactions to emotional stress, including a compulsive need to defecate. He exhibits masochistic tendencies and carries deep psychological scars symbolized by a battered, cat-like ego ball he seeks to bury. Additional traits include proficiency in origami, a fear of needles, possible allergies or social anxiety prompting frequent mask-wearing, and an interest in driving heavy vehicles or becoming a prison guard. He does not smoke.

His development consistently underscores a struggle with identity and self-worth, masked by detachment and feline affinity, rooted in the collapse of his adolescent attempt at reinvention.