Morph

Born a fully activated mutant, natural appearance was not welcomed by most
Stayed in a human form for most of their life, hardly ever serious
Joined the X-Men, killed during the team’s first encounter with the Sentinels
Reanimated by Mister Sinister, implanted a new personality that hated the X-Men;
Sought revenge on the team, separated & tormented them,
Eventually regained control, got treatment on Muir Island & returned to the X-Men