As far as teams, Quicksilver is known more for hanging around with the Avengers, but he’s also been a part of the X-Men, even though he started out as an enemy. With his superhuman speed, he was first introduced in 1964’s “X-Men” #4 by Stan Lee and Jack Kirby as a member of the Brotherhood of Evil Mutants, along with his sister Scarlet Witch, Toad, Mastermind and Magneto. He fought Xavier’s attempts to work with humanity, especially when he discovered he was the son of Magneto. Quicksilver went on to join the Avengers for many years, but didn’t stop there.
In “Uncanny X-Men” #304 (1993) by Scott Lobdell and Jae Lee, Quicksilver first crossed over to the side of the X-Men. In the story, Magneto returned for his revenge against the X-Men by bringing his fortress Avalon to Earth, bristling with alien weaponry. Quicksilver refused to join his father, siding with the X-Men instead. From there, Quicksilver would leave and join the X-Men, the Avengers, and manipulated Scarlet Witch into altering the universe in the “House of M” storyline, always following his own moral code.
In 1983’s “Uncanny X-Men” #169, Chris Claremont and Paul Smith created Callisto, the mutant leader of an underground gang of mutants. Callisto turned a Cold War bomb shelter into a hidden home beneath New York City, where mutants who were outcasts among humans could find peace. Many of the Morlocks were disfigured, but she wasn’t above disfiguring them herself if they weren’t when they arrived. She also kidnapped surface dwellers like the winged X-Man Angel. When the X-Men came after him, Callisto fought Storm for control of the Morlocks and lost, leaving herself humiliated and forced to submit to the X-Men.
In 1986’s “Uncanny X-Men” #211 (Chris Claremont, John Romita Jr. and Bret Blevins), the ruthless cyborg mutant hunters known as the Marauders slaughtered most of the Morlocks. Callisto was one of the only survivors, and was forced to join the X-Men to escape, survive and heal. From there, she became more of an ally of the X-Men instead of an enemy.