Kanichi Nishi, also known by his ring nickname Mammoth Nishi, is a pivotal supporting character who serves as Joe Yabukis closest friend, sparring partner, and eventually his trainer. He is first introduced as a juvenile delinquent who encounters Joe in a temporary jail, where the two fight and Joe emerges victorious. This initial conflict marks the beginning of a deep and lasting bond. Nishi later meets Joe again at a juvenile correctional facility, and it is there that their rivalry transforms into a genuine friendship.

Nishi undergoes one of the most significant personality transformations in the story. Initially, he is portrayed as arrogant and hot-headed, embracing his identity as a young troublemaker. However, witnessing Joe's intense boxing match against Toru Rikiishi in the detention center sparks a profound change in him, and he develops a deep respect for the sport. By the time he leaves the correctional system, he has adopted a calm and steady demeanor, finding work at a grocery store in the Namida Bridge neighborhood. This quieter life contrasts sharply with his delinquent past, but his passion for boxing remains alive through his connection to Joe. When Joe returns to the area, he is surprised to find Nishi at the Tange boxing gym, having joined specifically to learn how to box so he could stay by Joe's side and support his career.

As a boxer, Nishi is described as averagely good, lacking the raw, explosive talent of Joe or the polished skill of a prodigy like Rikiishi. Despite this, he proves to be an invaluable asset, notably becoming the only useful sparring partner Joe has throughout the entire narrative. His most notable trait in the ring is his reliability and toughness, providing Joe with the practical training needed to prepare for major fights. Nishi also harbors a romantic affection for Noriko Hayashi, the daughter of his employer at the grocery store, adding a personal layer to his life outside the gym. His own boxing career is cut short by an injury to his fist, a painful but decisive moment that forces him into an early retirement. At his retirement party, he confesses that he will always be proud of having dedicated his youth to the sport, challenging Joe to a final, symbolic match to close that chapter of his life.

Far from disappearing from the world of boxing, Nishi continues to be a cornerstone of Joe's support system. He eventually marries Noriko and becomes more dedicated to his work at the family store, yet he never abandons his role as Joe's most trusted confidant. He remains active in the corner during Joe's matches, working alongside the veteran trainer Danpei Tange to guide and encourage his friend. In this way, Nishi evolves from a rival to a loyal supporter, and ultimately into a trainer and manager, embodying a grounded, steadfast presence that balances Joe's wild and self-destructive nature.