A shapeshifter who can look like anyone, raised alongside Charles Xavier and asked by him to look like everyone else.
Mystique is the Fox films’ best idea about assimilation. She can look like anyone, so she is the mutant with the easiest possible life, and the films are about the fact that she does not want it. Charles raises her and gently teaches her to pass. Erik tells her not to, and that is the whole reason she goes with him.
Rebecca Romijn played her as almost silent; Jennifer Lawrence plays her as the centre of the later films. Days of Future Past hangs its entire timeline on one choice of hers, which is the most weight any of these characters is given.