Mystique may have made her first appearance in only 1978 (Ms. Marvel #16), but she’s a very popular villain and appears in many X-Men comic books. She’s a mutant with shape-shifting abilities. Luckily for her, Mystique’s shape-shifting rejuvenates her cells and stops her from aging. She’s also a highly trained terrorist and spy.
Mystique has a long and sordid history fighting with various versions of Magento’s Brotherhood of Mutants against both anti-mutant human organizations and the X-Men. Though through everything she’s done, Mystique seems to believe that she’s doing it for the good of mutant kind. She wants them to survive as a species.