Bantarō Sanbonmatsu is a student at Hontsuru Junior High School and a classmate of the protagonist, Jiyu Nanohana. He is the self-proclaimed leader of a small, tightly-knit group of friends who proudly embrace their unrefined and boisterous nature. The group initially consists of Kozaru and Ozaru, and is later joined by Shirō Ryūjōji. In terms of appearance, Bantarō is a young boy with black hair and brown eyes. He is often seen wearing a traditional gakuran uniform with his shirt left open, sometimes paired with a cap. His most distinctive feature is his shirt, which dynamically displays various kanji characters that change in real-time to reflect his current emotions, thoughts, or the topic of conversation.

Bantarō is defined by his intense and immediate romantic interest in Jiyu, having fallen for her the moment they first met. This infatuation becomes his primary motivation, driving him to make numerous, though consistently unsuccessful, attempts to confess his feelings to her. Each of these attempts is typically thwarted by persistent bad luck, awkward coincidences, or ill-timed interruptions. Despite his frequent failures, he remains undeterred and maintains an unwavering, if often misguided, dedication to Jiyu. His persistent proximity to her eventually leads him and his friends to discover her secret identity as the successor to the legendary swordsman Yagyu Jūbei.

Upon learning of her double life and the dangers she faces, Bantarō is motivated to offer his support against recurring enemies. However, his contributions are largely ineffective due to his complete lack of combat skills and his intellectual limitations. His impulsive decisions and general lack of intelligence are a source of frequent criticism from others, particularly concerning his future, as he is expected to one day take over his family's traditional sake brewery business. Bantarō's role in the story is primarily comedic, serving as a parody of a delinquent stereotype and a source of visual gags, particularly with his expressive shirt. This comedic role peaks in a notable incident where he and his friends mistakenly attack Jiyu's father after misinterpreting a plea for help.

In the second season, his narrative presence is significantly reduced. In a self-aware, meta-commentary on his own status, he and the character Shirō become conscious of their diminished roles, explicitly acknowledging their position as background figures in the ongoing story.