Kaoru Kumada, commonly called "Buta Gorilla," serves as a neighborhood bully. His parents selected the elegant, gender-neutral name "Kaoru," causing him embarrassment due to its feminine connotations and prompting his self-mocking nickname.

He consistently wears a blue-striped white t-shirt, brown pants, and a blue cap. As son of vegetable shop owner Kumahachi Kumada, he actively assists with family duties, displaying deep vegetable expertise and passion. Academically, he struggles with consistently poor grades and frequent math failures while claiming immunity to illness. He compensates with athletic excellence in sports, gaining popularity through mischievous antics.

His personality includes dimwittedness, habitual speech and action blunders, and susceptibility to correction by friend Tongari. He fears green caterpillars and initially bullies classmates and the robot Korosuke. His relationship with Korosuke evolves into a mix of bullying, underlying care, and situational teamwork. Similarly, interactions with Tongari blend bullying and comedic partnership.

Across the series, he shifts from deliberate antagonism to unintentional troublemaking driven by naivety, revealing his fundamentally good nature. Romantically, he pursues Miyoko before developing mutual affection for Sakurai Taiko, a sweet-natured transfer student who moves away; their bond persists through occasional visits. A time-travel episode leads to his eventual acceptance of the name "Kaoru" after meeting ancestors.

Notable quirks involve malapropisms, consuming vegetables with taste-altering furikake seasoning—later overcome by genuine appreciation—and vocal insistence he only falls ill during math tests. He frequently seeks help from frenemy Eiichi Kite, the protagonist.