Genos

Genos is a cyborg, rebuilt almost entirely from machine parts after a tragedy took everything from him. He hunts the man who destroyed his life while training to become strong enough for revenge. When he sees Saitama casually flatten an enemy that outmatched him, he declares himself a disciple on the spot.

He is the show's straight man in the truest sense: intense, formal, devoted to power rankings and self-improvement seminars, dropped into a world that keeps proving all of that beside the point. Kaito Ishikawa plays him with total earnestness, which is exactly what makes his devotion to Saitama, and his bafflement at his teacher's indifference to strength, so funny.