Genos

Genos is a cyborg hero who joined the Hero Association to find and destroy the creation that wiped out his hometown and family. After he lost his human body in that attack, he rebuilt himself as a nearly unstoppable machine fighter, and he earned S-Class rank almost as soon as he applied.

Genos takes being a hero extremely seriously. He trains constantly, studies every fight for ways to grow stronger, and throws himself into danger without much care for his own safety if it means getting closer to his goal. After watching Saitama effortlessly defeat an enemy that nearly killed him, Genos asked to become his student, and Saitama agreed almost by accident. Genos now lives with Saitama as both a roommate and a devoted disciple, constantly trying to copy his master's example even though Saitama's strength makes no logical sense to study.

The gap between Genos's serious, by-the-book nature and Saitama's lazy, joking attitude is a running source of humor in the series, but it also makes their bond genuine: Genos treats Saitama's casual advice like sacred wisdom, while Saitama, in his own low-key way, looks out for his overly intense student.