Boros-sama

Boros was the leader of the Dark Matter Thieves, a group of alien invaders responsible for the destruction of A-City. Like Saitama, Boros faced a self-imposed existential crisis, having become so powerful that no battle thrilled him.

Boros was the first on-screen opponent of Saitama that survived after getting punched, much to the hero's surprise. In fact, he is the very first of Saitama's opponents that took more than one punch to kill; Saitama even considered him to be the strongest opponent he had faced so far.