Gansaku Nanbu is a middle-aged man with a stout build, tanned, weather-beaten skin, and a scruffy beard. His most distinctive feature is a black eyepatch over his right eye, which is implied to have been lost as a consequence of unpaid debts. His typical attire consists of a pale cap, a yellow shirt, and an orange vest, a practical wardrobe that reflects his modest and financially strained existence. Once a legitimate boxer and the owner of his own gym, Nanbu now lives a nomadic life out of his truck after his gym closed down.

In terms of personality, Nanbu is a cynic and a pragmatist, shaped by years of operating in the harsh and unforgiving environment of underground Megalobox. He is often quick with a sarcastic remark and rarely displays his softer side openly, but his actions throughout the series reveal a more complex and caring individual. His approach to management and training is initially rooted in survival and self-interest, having traded integrity for the means to live. Despite his sometimes unscrupulous tactics and a personal struggle with gambling and alcoholism, he possesses a deep, buried well of knowledge about the sport of boxing. His primary motivation for most of the early story is to pay off a large debt accumulated from gambling on dog races, which forces him to manipulate fights for profit. This desperation ultimately leads him to sign his fighter, Joe, into the prestigious Megalonia tournament in a desperate bid for its grand prize.

Nanbu plays a central role as the trainer and manager of the protagonist, Joe. He begins the series as a cynical operator who orchestrates fixed matches for his fighter, seeing Joe as a tool to appease his creditors, such as the mob boss Fujimaki. However, when Joe defies orders and wins a match he was supposed to throw, their relationship is forced to evolve. Driven by debt and circumstance, Nanbu shifts from being an exploitative handler to a genuine, if gruff, coach. His bond with Joe transforms into a symbiotic partnership anchored in their shared struggle against a corrupt and class-divided system. Another key relationship is with his former student, Tatsumi Leonard Aragaki, a military boxer who returns from service to find Nanbu's gym closed, creating a long-standing resentment between them. He also has a complicated relationship with Sachio, a young boy who becomes Joe’s loyal supporter, with Nanbu eventually developing paternal instincts toward the group.

The character undergoes significant development from a compromised opportunist to a defiant challenger of systemic corruption. Initially presented as a washed-up has-been who abandoned his own student to gambling debts, Nanbu finds a form of redemption through his commitment to Joe. His journey is one of recovering his lost legitimacy as a trainer; he stops scheming and instead uses his buried tactical genius to guide Joe through the tournament. In later stages of the narrative, it is revealed that Nanbu is diagnosed with cancer, and his eventual death becomes a pivotal moment that cements his legacy of sacrifice. His defiant declaration, "I'll show you exactly what we cheaters can do!" perfectly encapsulates his transformation from a man who rigged the system for survival to one who rebels against it to prove his and his fighter’s worth.

Regarding his abilities, Nanbu is a highly skilled boxing strategist and tactician. Despite his years away from legitimate competition, his experience as a former boxer and trainer gives him an exceptional eye for a fighter's potential and an encyclopedic knowledge of boxing techniques. Throughout Joe's ascent in Megalonia, Nanbu devises clever fight plans that allow his underdog fighter to overcome physically superior opponents, demonstrating a cunning and resourceful intellect that was honed long before his fall from grace.