6) Quicksilver
Comic Books: Going back to the first X-Men comics, Quicksilver began as a villain serving under his father Magneto and alongside his sister Scarlet Witch as part of the Brotherhood of Mutants. The siblings later reformed, though, and joined the Avengers. Over the decades, Pietro Maximoff has continually crossed from the light and dark side – but always ends up as a hero in the end.
MCU: The biggest change the MCU made to Quicksilver was that – assuming he doesn’t come back to life at some point in the future – he isn’t a long-running member of the Avengers. Instead, Aaron Taylor-Johnson only appeared as a one-off enemy turned ally to the team in Avengers: Age of Ultron.
Pietro and Wanda were also stripped of their X-Men connections, meaning their father is not Magneto and they are no longer mutants. Their new origin as victims of experiments that gave them powers was later replicated in the comics.