10 Villains Who Became X-Men
5. Emma Frost
Emma Frost, as most people know, is a former White Queen of the Hellfire Club. Given her power set (which includes immensely powerful telepathy) and her attention to maintaining her appearance, she's exactly the right woman to rise through the ranks of high society. She also has an excellent knack for self preservation.
Emma has joined and worked against the X-men in intervals over the years. In Dark Reign she joined Norman Osborn's Dark X-men. Luckily it was a ruse by herself and Namor, but you can never be sure with Emma, who has been just as vindictive as an X-man as she was against them.
After Schism she sided with Cyclops all the way through Avengers Vs. X-men and beyond, so when Cyclops died from M-Pox, she continued his crusade to protect mutants at all costs. Much like her late partner, she leads her X-men along blurred lines of morality.