Banba operates as a private detective in Hakata, running his own agency while collaborating with police detective Shigematsu and informant Enokida. He simultaneously maintains a secret identity as the current "Niwaka Samurai," a vigilante targeting hitmen in Fukuoka. He also founded and plays for the grass-lot baseball team, the Hakata Tonkotsu Ramens.

Physically tall and lean, Banba typically wears a white sweater and old jeans. He often has a hunched posture, unkempt black hair, and brown eyes.

His personality is generally gentle and laid-back, showing little concern for personal appearance or domestic cleanliness, which sometimes causes friction with his roommate, Xianming Lin. He struggles with technology, often serving as a tester for Enokida's gadgets. While patient in most situations, he demands excellence regarding baseball, becoming intensely critical when coaching inexperienced players like Lin.

Banba's background involves significant trauma. Raised solely by his adoptive father, Kazuyoshi Banba, after his mother's death, his life changed irrevocably during high school. He witnessed an intruder stab his father. Attempting to intervene with a baseball bat, Banba was overpowered and knocked unconscious. He awoke to learn his father had died, while he survived severe injuries. During the attack, he glimpsed a man in a Niwaka mask subduing the assailant before calling for medical aid. Determined to find this figure, Banba dropped out of school and took a job at a host club in Nakasu, where the masked man was known to appear. Upon locating him—identified as Masataka, the then-Niwaka Samurai—Banba demanded training in lethal methods. Masataka agreed, and Banba spent several years as his apprentice.

Professionally, Banba investigates cases involving corruption and organized crime. During one major case involving Mayor Harada's conspiracy, he encountered the hitman Xianming Lin, initially sent to assassinate him. After Lin defected from the Kakyuu Association due to internal conflicts, Banba offered him shelter at his detective office, marking the beginning of an ongoing partnership. Banba later recruited Lin into his baseball team despite Lin's initial lack of knowledge or interest in the sport.

Banba's development shows a pattern of mentorship and concealed vigilance. As the Niwaka Samurai, he operates with calculated ruthlessness against criminals, as seen when he eliminated Kakyuu operatives threatening Lin. Yet in his civilian identity, he consistently provides Lin with stability and purpose, integrating him into both investigative work and baseball. Their relationship evolves through conflict and cooperation; for instance, six months after Lin joined the team, their persistent arguments during games concerned teammates, highlighting ongoing friction balanced by mutual reliance. Banba continues balancing his dual roles while navigating complex cases, such as uncovering marital infidelities or fulfilling contracts as the Niwaka Samurai, all while managing his responsibilities to the baseball team.

His character arc emphasizes adaptation to partnership, resilience against his traumatic past, and unwavering dedication to both justice and baseball. He appears in the context of "Hakata Tonkotsu Ramens," specifically in the episode titled "Hit and Run."