Karamatsu Matsuno stands as one of six identical Matsuno sextuplets. His childhood energy and cheerfulness masked an underlying bravado or "empty spirit" (karagenki), fueling impulsive actions like rushing into fights without thought for consequences. This recklessness frequently led to injuries, failures, and his tendency to become a scapegoat due to poor luck. He demonstrated occasional skills, sewing a large stocking for a Santa trap and showing artistic inclinations through painting, though these efforts were often sabotaged. His large appetite once resulted in consuming the family's entire rice supply, causing a severe stomachache. He frequently paired with Todomatsu, sharing misadventures where Todomatsu typically blamed him for their troubles.

During his teenage years (depicted in the -san movie), Karamatsu developed chronic acne and a subdued, worried expression. He wrestled with finding his individuality among his brothers and attempted to mediate their conflicts, though these interventions usually failed. His path toward a dramatic persona was influenced by his future self gifting him sunglasses and falsely promising a girlfriend.

As an adult, Karamatsu cultivates a confident, "charismatic" image, often dressed in blue clothing, a leather jacket featuring skull motifs, and sunglasses. He speaks in drawn-out, poetic language, believing himself popular and referring to hypothetical admirers as "Karamatsu Girls." Despite this self-perceived coolness, his actions frequently backfire, leading his brothers to label him as "painful" (Kusomatsu) and often ignore or mock him. Under pressure, his voice becomes higher-pitched, revealing underlying timidity. Easily intimidated, he complies when pressured, described by Choromatsu as "the nicest person in the world." Despite frequent mistreatment, he harbors deep frustration toward his brothers, occasionally expressing a desire to "kill them." His resilience allows him to bounce back after failures, maintaining his self-image. In later seasons, his role shifts slightly, gaining marginally more respect and focus in certain episodes.

His relationships with his brothers are complex. While he treats all siblings similarly, Ichimatsu displays persistent hostility, bullying him through physical attacks or destroying his possessions. This dynamic stems from Ichimatsu's jealousy of Karamatsu's confidence and hidden admiration, leading to a gradual softening after Karamatsu sacrificed his reputation to protect Ichimatsu. Karamatsu remains largely oblivious to the animosity, interpreting Ichimatsu's actions as a unique form of brotherly affection. His childhood bond with Todomatsu continues into adulthood but becomes less prominent.

In the "Ouma de Kobanashi" special centered on horse racing, Karamatsu appears alongside his brothers and supporting characters in skits involving betting.