Sidero serves as a combatant in the anti-Quirk terrorist group Humarise, operating under leader Flect Turn. A towering, heavily built man, his sharp features include slicked-back brown-and-black hair, a broad mouth, and disproportionately large, bulbous elbows. He dons a gray combat suit with truncated sleeves, cream boots, and a pale gold horned mask concealing his forehead and eyes. Black iron spheres affixed to each finger’s knuckles—excluding his thumbs—serve as both weapons and conduits for his Quirk. His Iron Ball Quirk allows him to produce and launch dense metal spheres from his knuckles, which he can expand in size and weight mid-trajectory to unleash devastating long-range assaults. Frequently paired with Beros on missions, he supplies her Longbow Quirk with ammunition while strategically bombarding targets to collapse structures and destabilize adversaries. Though outwardly committed to Humarise’s mission to purge Quirk users and avert human extinction, Sidero’s loyalty stems from opportunism. Having joined to evade execution—the group’s punishment for those bearing genetic anomalies—he prioritizes self-preservation. After Shoto Todoroki subdues him with ice during a border clash, Sidero swiftly bargains to expose Humarise’s secrets in exchange for mercy. During the *World Heroes’ Mission* crisis, he ambushes Izuku Midoriya and Rody Soul, pelting them with enlarged iron balls to reclaim a briefcase vital to dismantling Humarise’s Trigger Bomb scheme. Though his attacks ravage the surroundings, the duo evades capture. Beros later turns on him, firing an arrow through his leg to silence his cooperation with heroes, leading to his arrest. Before aligning with Humarise, Sidero leveraged his Quirk as a mercenary and guarded high-profile clients, cementing his expertise in combat roles. His documented involvement remains exclusive to the film’s events.