Koji Ishida is a deeply indebted middle-aged man who enters the high-stakes gambling events to clear the debts of his wife and son. He is defined by a timid and trusting nature, which makes him vulnerable in the harsh world of the gambling ship Espoir. Kaiji Itō is moved by Ishida's genuine concern for his family and rescues him from the forced labor of the ship.

Ishida's role in the story expands when he participates in the second round of the event at the Starside Hotel. He successfully wins a substantial sum in the Human Derby race. However, during the Electrified Steel Beam Crossing, his fear completely paralyzes him. Unable to proceed, he entrusts his winnings to Kaiji with the instruction to settle his family's debts. In a final act of immense dignity and resolve, he chooses to remain silent to avoid distracting Kaiji, falling to his death. This sacrifice deeply affects Kaiji, who admires Ishida's selflessness.

The impact of Ishida's story carries into the later arc. In the underground labor camp run by the Teiai Group, Kaiji meets his son, Hiromitsu Ishida, a man also trapped by debt. The memory of Koji Ishida's choice and Kaiji's promise to help his family serve as a powerful motivator in Kaiji's struggle against the system.

Ishida's development traces a path from a passive and desperate victim into a man who makes a heroic, conscious choice for his family. His primary motivation is his love and responsibility for them. His notable strength lies not in gambling or combat, but in a profound sense of honor and the quiet willpower he musters for his final act, leaving a strong moral legacy that inspires the protagonist.