Beast

Hank McCoy was the smartest kid in the room and the strangest: a boy with hands and feet built like a gorilla's, doing calculus while he swung from the rafters. Xavier recruited him into the original five, and Beast quickly became the team's voice, its acrobat, its brawler, and the man who explained what everyone had just fought. Then his own curiosity got him. A hormonal extract he tested on himself in the early 1970s remade him as a shaggy grey creature, the fur soon turning blue, and a later secondary mutation pushed him further still into something leonine and feline.

That body took him everywhere. Beast held down an Avengers seat in the 1970s, anchored the Defenders alongside Wonder Man, and reunited the original five as X-Factor. He spent the 1990s hunting the Legacy Virus and finally cracked it, saving thousands of mutant lives. Which makes what came later sting. On Krakoa he ran X-Force as the island's intelligence service and kept redefining acceptable losses, from mind-wipes to torture to a private army, until his own teammates hunted him. Wolverine ended him, and the Hank who walked away afterward was an earlier backup: the world's kindest genius, undone by the clone insurance he built himself.

Beast has been rebuilt more often than any other original X-Man. Lee and Kirby wrote him as a bruiser with a vocabulary; Steve Englehart's 1970s Amazing Adventures run turned him furry and funny, which got him onto the Avengers and into the Defenders. Louise Simonson's X-Factor put him back with his class of 1963, the 1990s books made him the mutant world's doctor, and Grant Morrison's New X-Men gave him the feline secondary mutation along with a scientist's arrogance. Benjamin Percy's X-Force took that arrogance to its logical end and made him the villain of his own title.