Legion
Real Name: David Haller
AKA: Jack Wayne. Cyndi. Jemail Karami.
First Appearance: Technically, New Mutants 26. But he was first shown in New Mutants 25 while Moira MacTaggert was making some notes on his case, and apparently doodled in the margins of the book.
What's His Problem? David is autistic and schizophrenic, which would be bad enough without having fantastic powers he can hardly control during his saner moments. As a young child, he was the lone survivor of a terrorist attack; striking out against his attackers, he absorbed the psyche of the leader of the terrorists, leading to his mental breakdown and his frequent struggle with multiple personalities.
Abilities: A high order psi who can steal astral forms and imprison them in his own mind, Legion is also a powerful telekinetic and pyrotec. And he is far, far more than all of that when sane and in complete control of his powers.
Heroes He Keeps Running Into: Early run-ins with the New Mutants aside, Legion's feelings of childhood abandonment means he can't stay away from his father, Charles Xavier. When he achieved a moment of relative lucidity, he decided to help his father's dream of mutant/human peace by going back in time and murdering the one person Legion thought most destructive to Xavier's vision. Alas, Legion killed the wrong person and ended up erasing himself from continuity.
People Who Think He's Not So Bad: Moira MacTaggert, Legion's mother Gabrielle Haller, and his father Charles Xavier never stopped trying to help him, even when Legion was on the verge of destroying everything they knew.
Most Despicable Act: Although he snagged the New Mutants' astral forms to be toys in New Mutants 26-28, he meant no malice. However, he meant a great deal of malice when he created a vortex to travel to the past and murder Magneto, thereby removing the chief obstacle between his father's dream of unity between human and mutant. Accidentally killing Xavier instead, Legion set up the terrible atrocities that would occur during the Age of Apocalypse. Whether Legion still exists or not since the re-establishment of the time-line has never been covered, although my guess is that he is in a convenient coma someplace, just waiting to be revived and create more havoc.