Karamatsu Matsuno is the second-born of the Matsuno sextuplets. His name comes from the Japanese word for the larch tree, "karamatsu" (唐松), with additional puns on "kara" meaning emptiness, referencing early character notes that used descriptors like "empty spirit" (karagenki) or "empty head" (karappo).

As a child, Karamatsu was cheerful, carefree, and possessed the most energy among his brothers, though this energy sometimes manifested as bravado. His impulsive nature frequently led to injuries or trouble for himself and others. He was also described as a bit of a crybaby and was often unlucky. Though not particularly intelligent, he showed artistic inclinations in painting and sewing. He had a large appetite and was somewhat tidy and orderly compared to his siblings. In childhood groupings, he was most often paired with Todomatsu, and the two were frequently shown getting into mischief together.

During his teenage years, Karamatsu developed chronic acne and grew more timid and subdued. He struggled to find his place within the group and attempted to mediate his siblings' conflicts, often meeting with failure. His path toward a dramatic persona began when his future self gave him sunglasses and falsely told him he would have a girlfriend.

As an adult, Karamatsu adopts a flashy and self-assured demeanor. He believes himself to be cool and popular, often speaking in drawn-out, poetic fashion and incorporating English words into his sentences. He frequently wears a blue hoodie with the sleeves rolled up, blue jeans, and brown boots, though his signature outfit is a black leather jacket with a skull on the back, a skull belt buckle, and blue sunglasses. He accessorizes with items bedazzled or featuring his own face. Despite his confident exterior, he remains oblivious to social cues, and his attempts to be cool usually backfire, causing others to find him "painful." His brothers frequently ignore, dismiss, or physically abuse him, and he is sometimes derogatorily called "Kusomatsu" (Shittymatsu), particularly by Ichimatsu.

Beneath this flamboyant persona, Karamatsu is timid, easily intimidated, and desperate for his brothers' approval. He cares deeply for his family and will perform dangerous or insane acts for their happiness, even if they go unappreciated. He has a fragile ego, and his voice becomes higher when nervous, suggesting his deep voice may be an affectation. His behavior is interpreted as a coping mechanism developed from his insecure teenage years, an act to become the confident person he wished to be.

Over the course of the series, his character evolves. In later seasons, his flashy narcissism becomes more subdued; he wears his flamboyant clothes less often, reads more, and avoids involvement in his brothers' schemes, indicating a decline in mental health and growing depression. An episode centered on a pizza order highlights his frustration with his brothers, who have historically ignored his feelings and only engage to criticize him.

Karamatsu's relationships with his brothers are complex. Osomatsu finds him funny rather than cool. Choromatsu finds his behavior embarrassing. Ichimatsu is openly hostile and aggressive toward him. His relationship with Jyushimatsu is neutral but distant, and Todomatsu is often embarrassed by him. Despite this, Karamatsu consistently shows care and love for all of them.

He remains artistically inclined, playing the guitar well and possessing painting skills. His favorite food is karaage and meat, and his favorite drink is whiskey, though he often drinks barley tea instead while claiming it is whiskey. He has a stated fetish for panties. He knows how to cook fried rice, Osomatsu's favorite food, and judging by the movie, he does not know kanji well.

In official merchandise, his associated animal motif is a tiger, and the physical manifestation of his self-consciousness is depicted as a glossy, translucent blue orb. In an alternate future depicted in a one-shot story, he married a greengrocer's daughter.