Boros, also known as Lord Boros to his subordinates, is the leader of the Dark Matter Thieves, a formidable group of alien invaders. His origin lies on a planet with an extremely harsh environment, which forced his species to evolve incredible physical capabilities and the greatest regenerative ability in the universe. Boros was born as an exceptional being among his already powerful race, possessing physical prowess and latent energy that far surpassed all others.

Driven by an existential crisis caused by his own overwhelming power, Boros became deeply arrogant and prideful. Having never encountered an opponent who could challenge him, he roamed the cosmos for decades ravaging worlds, not out of a desire for conquest, but out of a desperate search for a battle that could excite him. This boredom and craving for a worthy opponent is his primary motivation. An alien seer once told him a prophecy that on a faraway planet, he would find a being who could fight him on equal footing, giving him the enjoyable fight he always desired. This prophecy gave him purpose, leading him to travel the universe for twenty years to reach Earth.

Boros serves as the central antagonist of the Alien Conquerors arc. Upon arriving above A-City, he orders his ship to bombard the city, destroying nearly all of it in an instant to draw out any powerful opponents. He demonstrates a callous indifference to the destruction and loss of life his invasion causes. However, he also shows a certain code of conduct and respect for strength, reassuring his subordinates and greeting Saitama properly when they finally meet. His key relationship is solely with Saitama, whom he immediately identifies as the opponent from the prophecy. His subordinates, including the powerful Geryuganshoop and Groribas, serve as his lieutenants but are quickly defeated by Saitama, leaving Boros to face the hero alone.

The battle with Saitama represents Boros's entire character arc. Initially confident, he is shocked when Saitama's first punch destroys his restraining armor, which he wore to suppress his immense power. As the fight progresses, Boros unleashes his true strength, moving through several forms and pushing his abilities to their absolute limit. He is the first enemy Saitama acknowledges as being strong, and the fight forces Saitama to use a Serious Punch. Despite Boros's own power and determination, he gradually realizes the vast, insurmountable gap between them. In his final moments, after being mortally wounded, he finds peace, not in victory, but in the satisfaction of having fought a battle as exciting as he had always hoped. He admits that the prophecy was likely a lie or that Saitama was simply too strong, acknowledging his defeat with a warrior's dignity.

Boros possesses a wide array of notable abilities. He has immense superhuman strength, speed, and durability, being able to knock Saitama through massive pillars and even kick him all the way to the moon. His most defining trait is his unparalleled regeneration, allowing him to regrow limbs in seconds and completely reform his body even after being blown apart by a barrage of punches. He can project powerful energy blasts from his body's core. His trump card is Meteoric Burst, a form where he converts his vast latent energy into propulsive power, granting him explosive speed and strength that exceeds the limits of his biology, though it places a tremendous strain on his body. His ultimate attack is the Collapsing Star Roaring Cannon, a technique where he releases all of his remaining energy in a single colossal beam, an attack with enough power to completely destroy the Earth's surface or even the entire planet.