Boros, also called Lord Boros by his subordinates, 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. His intergalactic travels in search of a worthy opponent led him to Earth, where he was killed by Saitama.

Boros is the main antagonist of the Alien Conquerors Arc. Boros was a very tall cyclops-like alien with light-pink, spiky hair and bangs that fell over his face. He had sharp teeth and pointed ears lined with a row of gold earrings. He had one large blue eye and light blue skin with dark blue crack-shaped markings that started from his eye and covered most of his body. He wore a set of armor with spikes on the shoulder and forearms and wore a pair of matching curved boots. In the manga, his skin in his sealed form was purple with darker purple stripes on his face. He also had a red orb on his chest with purple markings on his stomach and arms. He wore a pair of white harem pants, held by a magenta-colored sash with silver-lined purple blades hanging from it.