Kanichi Nishi, better known as Mammoth Nishi, is a supporting character in Tomorrow's Joe 2. He was born with a large, imposing physique that earned him the nickname Mammoth. Nishi first appears as a juvenile delinquent who encounters the protagonist Joe Yabuki in a detention center. Initially depicted as an arrogant and ruthless thug, he orders other inmates to assault Joe. However, after being defeated and later meeting Joe again in reform school, his personality begins to shift. He reveals a much softer and more emotional side, often crying openly at things that move him. He becomes friendly, polite, and deeply loyal, a stark contrast to his earlier demeanor.

His motivations evolve from a desire for dominance and survival in the juvenile system to a genuine friendship with Joe and a dedication to boxing. He takes up the sport seriously after Joe inspires him, and together they train under Danpei Tange. Nishi's role in the story is multifaceted. He serves as Joe's first real sparring partner and closest friend, a grounding presence who balances Joe's wilder tendencies. After their release from detention, the two live together in Danpei's shack under a bridge, which doubles as their boxing gym. Nishi also works at the Hayashi family grocery store, where he eventually discovers a talent for retail work.

Key relationships define his arc. His bond with Joe is the most important; they start as rivals in detention but grow into inseparable friends and training partners. Nishi also forms a strong relationship with Danpei, who trains him. Through his job, he meets Noriko Hayashi, whom he later marries, settling into a conventional family life. His relationship with the Hayashi family provides him with stability and purpose outside the ring.

Nishi's development is one of the most complete in the series. He transforms from a juvenile delinquent into a calm, responsible, and successful adult. As a boxer, his career is moderate compared to Joe's. Competing as a heavyweight, a class with limited opportunities in Asia, he achieves some success but struggles with maintaining weight and suffers a hand injury that forces him to retire. He then becomes Joe's co-trainer, using his deep understanding of Joe's style and weaknesses to support him from the corner. His sharp observation skills are noted; he can identify Joe's fatal flaws and recognize his opponent's motivations.

Notable abilities include his raw physical strength, which made him a formidable presence in the ring, and his tactical mind as a trainer. Even in retirement, his ability to analyze fights and guide Joe proves invaluable. Nishi's emotional sensitivity, often seen through his tendency to cry, is not a weakness but a sign of his deep capacity for empathy and loyalty. His arc represents a life rebuilt through hard work, friendship, and dedication, making him a stabilizing and heartwarming presence in the story.