Sidero is a tall man with slicked back brown and black hair, an extremely wide mouth, and large bulbous elbows.
He wears a gray combat suit of similar design to Rogone and Beros', though his version has sleeves going to his upper arm. Accompany his suit are cream-colored boots, and pale gold horned mask that obscures the upper half of his face. All of his fingers barring his thumbs have black iron balls on his knuckles.
Like most Humarise members, Sidero is devoted to carrying out Flect's plan. Although he presumably only joined the cult to avoid being killed for having a Quirk, as he was quick to offer information once captured by Heroes.
Iron Ball (アイアン・ボール Aian Bōru?): Sidero's Quirk allows him to create and detach iron balls from his knuckles, which can then be enlarged in size and mass. He can detach them to provide ammo for Beros' Quirk.
- He is referred to as the Iron Ball Villain (鉄球ヴィラン Tekkyū viran?).
- Sidero English voice actor, D.C. Douglas, also voices Untenmaru Kurumada.
- Sidero was a security guard for a VIP before being recruited into Humarise.