Emma Frost's Heel Face Turn is proving permanent. However just like Jean, Emma suffers from the Never Live It Down trope. The story that cemented her position as a central X-Man was also the story in which the readership was first led to believe she betrayed the X-Men, just to discover later that she was being psychically manipulated by Cassandra Nova. Before having her own mind trapped inside an indestructible box, Nova had left a psychic impression of herself in Emma's mind, which was subtly manipulating her. That impression slowly twisted Emma's perception of herself, exploiting her survivor's guilt and making her believe she could never redeem herself. In the end, it was shown that Emma was willing to sacrifice herself so that Nova's plan wouldn't succeed. Besides that story, the only other time Frost fitted this role was during Dark Reign, when she joined Norman Osborn's Cabal, as part of her and Cyclops' plan to later make a fool of Osborn and guarantee the safety of mutantkind. Also, while part of the fanbase and some writers believe Emma is still not trusted by her teammates, there are many evidences pointing the opposite. For one, she's been banking the X-Men for quite some time now, and they're not uncomfortable with that. More importantly, she's been the main Cerebro user ever since Professor Xavier left, and the X-Men are ok with that (Cerebro is a machine able to boost one's telepathic power to impressive levels. By letting Emma use it, the X-Men are leaving their minds - and those of thousands of innocents - completely vulnerable to her powers, which says a lot).
For everyone but the three people who might remember that issue of ROM, it was the fact that her powers were driving her slowly insane and she was realizing that Mystique was both unable and unwilling to help, with a little nudge from Mastermind (who wanted to put the screws into Mystique by inducing her beloved foster daughter to run away) that did the trick.
There's also Sunfire, Banshee, Magneto himself, Marrow...
Magneto is the founder and leader of this continuity's version of the X-Men.
Sabretooth goes from a villain with standards to a genuine believer in Magneto's creed, a would-be atoner, and a surrogate father figure to Blink and Wildchild.
Sauron and Toad, both minor villains, are on the heroic side and loose allies of the X-men here.
Mystique ferrets out information for Magneto and runs a secret escape line for smuggling humans out of Apocalypse's genocide-zones... although she also remorselessly skims wealth from her "customers" as well.
Wolverine: Originally an antagonist sent to kill the Hulk (who himself is a big spinning heel-turn himself... not that both characters don't have their reasons...).
