The shape-shifting blue woman who frequently appears in X-men comics, cartoons, and movies is Mystique.

Mystiqe was already in the comics

Mystique has been portrayed as both a villain and an anti-hero in Marvel comics. She is a complex character with a shifting moral compass, sometimes aligning herself with the X-Men and other times with villainous groups like the Brotherhood of Mutants.

The character's age is uncertain, and in comics she is more than a century old. However, in the film X-Men First Class (2011) she is shown as being slightly younger than Charles Xavier.

The starlet had a certain mystique about her.