Juggernaut, or Cain Marko, is literally the red-headed step-brother of Professor Charles Xavier, founder of the X-Men. Due to a magic gem he found while he and Xavier were serving in Korea he became the magical Avatar of the demon-lord Cytorrak, an unstoppable* 'juggernaut' with incredible strength and invulnerability. He wears a magic helmet that keeps out psychic powers; it's meant to be unbreakable, but literally every fight he has with the X-Men ends with the helmet breaking and one of their fifty telepaths frying his brain.
(*Character may be unstoppable in name only, and may in fact be frequently stopped by the likes of Spider-Man.)
