Legion

Charles Xavier fathered a son with Gabrielle Haller and did not know it for years, which is the least of David Haller's problems. As a boy he survived a terrorist attack that killed the adults around him, and his emerging mutant power did something no child's mind could hold: it consumed the attacker's consciousness whole. David went catatonic. What woke up was not one person. His mind fractured into a crowd of distinct personalities, each one holding a different mutant ability like a weapon in a locked room, and control of the body goes to whichever of them shouts loudest.

That makes him one of the most dangerous mutants alive and one of the most sympathetic. He is an Omega-level talent whose illness is his power set, capable of telepathy, telekinesis, pyrokinesis and, at the far end, rewriting reality itself. When the Shadow King took root in that fractured psyche, destroying him cost Xavier dearly. Worse followed. Trying to save his father's dream, David travelled back in time to kill Magneto, killed Xavier instead, and erased the man whose dream he was defending, creating the Age of Apocalypse. Every good intention David has ever had came with a body count.

Bill Sienkiewicz's expressionist New Mutants art is inseparable from Legion's debut in 1985, because the character needed to look like nothing else on the shelf and did. Claremont used him first as a horror story about Xavier's blind spots, then Marvel used him as a plot bomb: Legion Quest in 1994 and 1995 sent David back to murder Magneto, produced the Age of Apocalypse instead, and replaced every X-title in publication for four straight months. Simon Spurrier's X-Men Legacy in 2012 finally treated him as a protagonist, a young man negotiating with his own mind rather than a walking catastrophe, and the Krakoa era's Legion of X built on exactly that reading.