Ichirou Hiiragi is a player in the Darwin's Game who appears early in the story. He is a man in his forties with neatly combed black hair and some facial hair, and during the game he wears a full body suit that conceals his features, making him appear mysterious. Outside of combat, he dresses casually in a white shirt, dark pants, and an apron.

He suffers from a heart condition, and his daughter, Suzune Hiiragi, is a twelve-year-old girl with a weak heart who has been living in a hospital. Ichirou's primary motivation for participating in the Darwin's Game is to earn enough money to pay for Suzune's heart surgery. This reveals a deeply caring and selfless personality; he is a kind man who prioritizes his daughter's well-being above his own life. He is willing to kill other players to achieve that goal, but his actions are driven by desperation rather than cruelty.

His sigil, or special ability, allows him to control plants. He uses this power to set traps, create strong armor, and fight effectively at close range. He is a formidable opponent, nearly defeating the protagonist Kaname Sudō in their battle. Despite his strength, he ultimately dies in his encounter with Kaname, an event that occurs early in the story and has a significant emotional impact on later developments, particularly on his daughter.

Ichirou's role in the story is that of an early antagonist who serves as a tragic example of how the Darwin's Game corrupts and consumes even well-intentioned people. His death leaves Suzune orphaned, and she later becomes a supporting character, joining the Danjou Boxing Club and forging her own path within the game. Ichirou's brief appearance establishes the high stakes and moral complexity of the game, as his genuine love for his daughter contrasts with the violence he is compelled to commit.