Raven Darkholme is a blue skinned, shapeshifting mutant.
Raven Darkholme was born in Hamburg, Germany, and orphaned at four years old. She was adopted by an agent of HYDRA who raised her to become an assassin. She became a Hydra agent, after getting out of high school.
While on a mission to steal some toxins and biochemicals for Hydra, she was attacked by S.H.I.E.L.D. agents. She accidentally fell into a vat of chemicals and was badly burned, then taken into the custody of Shield. After an attack by Hydra, she was freed from a Shield base in Alaska, and taken back to Hydra's HQ.
While recovering from her burns from the chemical factory, she wakes up one day, completely recovered and with no burns anymore. But, she sees that her skin has turned blue and finds that she has the power to shapeshift. She reads some of the records of the Hydra agent that adopted her, and finds out that she had occasionally had fits of rage as a child, where her skin would turn blue. She discovers that she is a mutant, but her abilities did not unlock until after the incident at the chemical factory.
She later goes fugitive and quits being a Hydra agent, after she hears that some Hydra scientists want to experiment on her, for her abilities. She goes under the codename "Mystique" and travels the world, shapeshifting and using aliases.