Kanichi Nishi, better known by his ring name Mammoth Nishi, first appears in Tomorrow's Joe as a juvenile delinquent serving time in a reformatory. He meets the protagonist Joe Yabuki in a juvenile jail, where Nishi initially acts as an arrogant and ruthless cell leader who orders other inmates to assault Joe. Joe defeats him in a fight, and when the two later meet again at a correctional facility, Nishi reveals a more anxious and cowardly side, and the two become friends. This early encounter establishes the foundation of a relationship that shifts from hostility to camaraderie.

Nishi's background marks him as a young man from a wealthy family who abandoned a privileged life to pursue boxing. His imposing physical size and strength earn him the nickname Mammoth Nishi, and he is both feared and respected inside the ring. Despite this fearsome reputation, his personality undergoes a significant transformation over the course of the story. The arrogant thug gradually gives way to a gentle, emotional, and soft-hearted individual, known for frequently crying at things and for being friendly and polite to everyone he meets.

His motivations are closely tied to his friendship with Joe. Nishi enters professional boxing alongside Joe, training under the former boxer Danpei Tange and living with Joe in a humble shack under a bridge that serves as their training gym. While Joe rises to become the Asian bantamweight champion, Nishi's own boxing career achieves only moderate success. He competes as a heavyweight, a weight class with few opportunities on the Asian boxing circuit, and briefly finds more success after moving down to middleweight. However, he struggles to maintain his weight due to a tendency to overeat, and a hand injury eventually forces him to retire from active competition.

Nishi's role in the story evolves from Joe's rival and sparring partner to his closest friend and, later, his co-trainer. After retiring from boxing, he takes on a supporting role behind the scenes, helping to train Joe for his most important matches. His relationship with Joe is the central personal bond in his life; he is fiercely loyal to Joe and proud of his friend's accomplishments, as shown when Joe delivers a congratulatory speech at Nishi's wedding.

Beyond the ring, Nishi finds stability and purpose in ordinary life. He works at the Hayashi family grocery store, where he discovers a talent for retail work, and eventually marries Noriko Hayashi, the store owner's daughter, settling into the role of a conventional family man. This development stands in stark contrast to Joe, who risks everything for boxing alone. Nishi's transformation from a delinquent into a calm, reliable worker and husband represents one of the story's more grounded arcs.

In terms of abilities, Nishi is a physically powerful boxer whose size and strength make him a daunting opponent. His most notable skill lies not in championship-level boxing, however, but in his later role as a trainer, where he proves effective at coaching and supporting Joe from the corner. His emotional sensitivity, which once seemed a weakness, becomes part of his strength as a loyal friend and mentor. He matures from a man whose pride is tied to his own fighting record into someone who finds fulfillment in helping others succeed and in building a peaceful life outside the sport.