Quicksilver

Pietro Maximoff arrived as a villain, sprinting for Magneto's Brotherhood alongside his sister Wanda because they owed the man a debt and had nowhere else to go. Within two years he was an Avenger, one third of Cap's Kooky Quartet — the reboot lineup that proved the team could survive losing its founders. Pietro has been rude to almost everyone in the Marvel Universe since. The arrogance is armour: abandoned as an infant, raised by strangers, shaped by a father figure who used him, he decided early that being fastest was the same as being safe.

His worst decisions are the interesting ones. He married the Inhuman Crystal and moved to Attilan, becoming Marvel's most impatient royal in-law. He is the one who whispered House of M into Wanda's ear, rewriting reality out of love and panic, then compounded it in Son of M by stealing the Terrigen Mists to give depowered mutants their gifts back. Marvel spent the 2010s retconning Magneto out of his parentage; the Krakoa era walked part of that back. Through all of it Pietro keeps volunteering for a hero job nobody thinks he deserves.

Lee and Kirby created the Maximoff twins as reluctant Brotherhood members, and Marvel almost immediately realised they worked better as heroes — an early demonstration that a Marvel villain is often just a hero with a bad first appearance. Quicksilver's publishing life has been unusually unstable since: Avenger, Inhuman consort, X-Factor member, occasional antagonist, House of M architect, Inhumans supporting player. Marvel's mid-2010s decision to sever both twins from Magneto and from mutantkind was driven as much by branding as by story, and the Krakoa era quietly restored part of what it took away.