Boros

Boros, also called Lord Boros by his subordinates, was the leader of the Dark Matter Thieves, an alien invasion force responsible for A-City's destruction. Known as the Dominator of the Universe after countless battles across the cosmos, he suffered the same existential problem as Saitama — grown so powerful that no fight could satisfy him — and his search for a worthy opponent finally led him to Earth, where Saitama killed him.

Arrogant and prideful because of his overwhelming power, Boros roamed space seeking someone capable of matching him, growing callous toward the destruction his invasions caused along the way. Even so, he showed real moments of courtesy, reassuring a worried Geryuganshoop and warmly greeting Saitama upon arrival. As their fight dragged on and Saitama kept withstanding every attack, Boros's respect for him only grew, and he ultimately met defeat at peace, fulfilled simply by having found the thrilling battle he'd spent decades searching for.