Mammoth Nishi, whose real name is Kanichi Nishi, is introduced as a formidable and intimidating presence who initially antagonizes the protagonist Joe Yabuki. When they first meet in a juvenile detention center before Joe's trial, Nishi acts as a boss figure who harasses the other inmates, but his aggression is quickly and decisively shut down by Joe. Following this encounter, both are sent to the same juvenile correctional facility. There, Nishi undergoes a significant shift in behavior, appearing timid and careful not to provoke the senior inmates, a stark contrast to his earlier confidence. Despite this, he demonstrates loyalty and courage by attacking other inmates to help Joe during an escape attempt, an act that forges a deep bond between them. Nishi is impressed by Joe's unyielding spirit and reckless bravery, and he becomes the only person Joe truly befriends during their time incarcerated.

Upon his release, Nishi chooses to turn his life around. He begins working diligently at a grocery store and, inspired by the boxing he witnessed in the detention center, joins the Tange gym under the coach Danpei Tange. As a boxer, he adopts the ring name Mammoth Nishi, a reference to his large stature. While he is a capable fighter with a respectable record of ten wins, all by knockout, his career is hampered by his weight class. As a heavyweight, he struggles in a Japanese boxing scene that offered few opportunities for larger athletes, forcing him to endure difficult weight management. Despite being a solid boxer and the only effective sparring partner for Joe throughout the entire story, his professional career is cut short by a hand fracture that forces him into a painful but accepting retirement. At his retirement ceremony, he expresses pride in having dedicated his youth to boxing, even sharing a final sparring session with Joe.

Following his retirement, Nishi successfully transitions to a stable and prosperous life outside the ring. He marries Noriko Hayashi, the daughter of the grocery store owner where he works, and his dedication leads him to open several branches of the store. In this way, he becomes one of the most successful characters in the narrative, representing a path of social rehabilitation and conventional success that contrasts sharply with Joe's all-consuming and self-destructive boxing career.

Throughout the story, Nishi serves as Joe's closest friend, a pillar of support, and a man of sound judgment. As Joe's second in his most critical matches, he shows a sharp and perceptive side, often being the one to notice when something is wrong with Joe, such as his inability to punch after the traumatic death of a rival. He is remarkably generous and loyal, once giving Joe his entire paycheck without hesitation. Even after his retirement, he remains an integral part of Joe's corner, continuing to train him and work as a second during fights alongside Coach Danpei. His key relationships include his deep friendship with Joe, his mentorship under Danpei, and his loving marriage to Noriko. Nishi’s development from a juvenile delinquent into a calm, responsible, and successful adult forms a complete and traditional character arc, making him a grounding presence in the story and a testament to a life rebuilt through hard work and loyalty.