Raven Darkholme alias Mystique is a shape-shifting mutant villain.
At some point prior to her first appearance (presumably during the events in first episode), she was trapped inside a DISK, which then ended up in the hands of Red Skull. After stealing a biocode for himself from the Celebrity Five, Red Skull released Mystique from her DISK and promised her freedom in return for a favor; she had to steal the DISKs containing the villains Baron Zemo, Abomination, Tiger Shark, Graviton and M.O.D.O.K. from the Avengers’ base.
Mystique easily succeeded in this mission by impersonating Pepper Potts. When the Avengers and their partners discovered the theft, they, along with the Japanese hero Sunfire, chased after her. This turned out to be a good thing, because after Mystique delivered the DISKs to Red Skull’s henchman Crossbones, he revealed he had been ordered to kill her. He knocked her out with a gas weapon and was about to shoot her when Sunfire arrived and interfered. Mystique was not seen again after the Avengers’ fight with Red Skull and his new Masters of Evil. Presumably, she slipped away during the battle.