Who am I? That, my dear, is an excellent question. Though not one easily answered.
Mystique (real name: Raven Darkholme) is a major villainess from the X-Men comics and media. She is a somewhat treacherous ally of Magneto who has worked as part of the Brotherhood of Evil Mutants but has also taken over - being capable of even more radical thinking than Magneto at times.
She is one of the X-Men's most deceptive and cunning opponents, happy to do what it takes to get the upper-hand and ensure mutant supremacy - this ruthlessness has been displayed when she attempted to kill her own adoptive daughter Rogue and son Nightcrawler on numerous occasions.
Mystique is also quite happy to switch sides if it is in her interests and this combined with her formidable shapeshifting abilities serves to make her a very dangerous opponent.
In the movie X-Men, it was revealed that Mystique's own family tried to kill her for being a mutant. Later, she joined Magneto and became his trusted right-hand.
In the comic books however Mystique's past is much more mysterious, her exact age is never revealed but she is known to have been alive and active in the late 19th century and during her travels would have a relationship with Sabretooth, ultimately producing Graydon Creed - oddly enough despite his origins Graydon would grow to be a human rather than a mutant and indeed would be one of the X-Men's greatest enemies as the leader of the anti-mutant organization Friends of Humanity - at any rate Mystique soon broke up with Sabertooth and would continue on her tour of the world, eventually giving birth to another son, Nightcrawler, by the demonic mutant Azazel but she would abandon this child as well.
Ultimately Mystique would gain a final child by adopting Rogue and settle into a criminal career as part of the Brotherhood, where she would grow to be one of the X-Men's most hated and recurring foes.
Powers and Abilities
Mystique has the ability to shape-shift into anyone she chooses, even those of the opposite gender and is well versed in all manner of both armed and unarmed combat.
These powers combined serve to make her a stealthy opponent capable of tricking many, however it is not as effective against Wolverine, as she can not disguise her scent without aid - she also suffers from the limit that she can not shapeshift into non-humanoid forms.
In the comic book canon only she displays the additional powers of regeneration and immunity to age, said to be connected to her ability to shapeshift.
Mystique has a strange family. She and Sabretooth had a relationship though she shapeshifted into someone else. She ends up having a son named Graydon Creed. She also had another son named Kurt Wagner a.k.a. Nightcrawler, from a love affair with the extra-dimensional warlord, Azazel. Mystique also adopted Rogue when she was very young. Mystique had a lesbian relationship with her life long friend Irene Adler AKA Destiny, who helped raise Rogue. In films she is Charles Xavier's adoptive sister and a former lover of Magneto
Supervillain Origins Mystique
Watchmojo.com explores the origins of Raven Darkholme, otherwise known as Mystique