Saitama (サイタマ, Saitama) is the main protagonist of the series and the titular One-Punch Man. He is currently the most powerful being to exist in the series. Saitama faces a self-imposed existential crisis, as he is now too powerful to gain any thrill from battle.

Initially just a hero for fun, Saitama later registers to be a professional hero for the Hero Association upon realizing that nobody recognizes him as a hero, and defends his home in Z-City from monsters, villains, and other threats. Under the Hero Association, he is assigned the hero name Caped Baldy (ハゲマント, Hagemanto; Viz: Bald Cape) and is currently A-Class Rank 39.