Joichiro Nishi, full name Joichiro Nishi, is a middle-school student who becomes one of the earliest veterans of the Gantz program, having been recruited more than a year before the main story begins. He originally died after falling from a rooftop while fleeing from police following a shoplifting incident. His entry into the Gantz room occurs at that moment of death. Nishi’s background is marked by severe bullying: his classmates ostracized him and ultimately threw him out of a classroom window. In retaliation, he killed all of them except for one girl who had a crush on him and did not participate in the harassment. After this massacre, civil armed forces were sent to eliminate him; Nishi managed to kill many of them but was heavily wounded, only surviving because a new Gantz mission summoned him away.

Nishi’s personality is defined by selfishness and a Social Darwinist worldview. He believes that survival belongs to the strongest and shows little concern for the lives of others. He is calculating, pragmatic, and willing to abandon teammates to ensure his own survival. He is reluctant to share information about Gantz or himself, though he secretly operates a website where he posts details about his missions, carefully avoiding the rule that would cause Gantz to explode his head for revealing the game to outsiders. Gantz itself addresses him as Mr. Nishi, a contrast to the dismissive way he treats other participants.

His primary motivation is self-preservation and earning one hundred points to be freed from the game. He views the missions as a means to that end and does not form emotional attachments. In combat, Nishi is not a particularly strong fighter; he relies on stealth, ambushes, and strategic thinking rather than direct confrontation. He is resourceful and adapts to the brutal rules of the Gantz world.

In the story, Nishi serves as an experienced but antagonistic presence among the players. He is killed early during the Tanaka and Suzuki alien mission, but after six months of being stored in Gantz’s memory banks, he is revived by another player, Kaze, at the request of Kei Kurono. Kurono hopes Nishi can provide crucial information about Gantz and the upcoming catastrophe. After his revival, Nishi is forced into a more cooperative role, though his cynical nature persists. He later reveals the hidden catastrophe countdown on the Gantz sphere, which signals an impending global alien invasion.

Key relationships include a tense dynamic with Kurono, who initially distrusts Nishi but later relies on his knowledge. Nishi maintains distance from most other participants, viewing them as tools or liabilities. His development is limited: he remains largely self-interested, but he does become a source of vital intel and eventually participates in the final battles against the alien invasion. His notable abilities are not physical but intellectual—he is a quick thinker, a careful planner, and possesses extensive knowledge of Gantz’s mechanics and rules. He uses standard Gantz-issue weapons and suits but relies more on clever tactics and environmental awareness.