Who she is: Callisto
Her power: Callisto has super-speed and can track other mutants and sense their powers.
Her story: Callisto is the leader of the Omegas, a renegade mutant group that allies with Magneto’s Brotherhood in X-Men: The Last Stand. The development of the mutant cure pushes Callisto and her fellow Omegas out of the shadows and toward Magneto’s human-hating fanaticism, and the two teams join forces to take down Worthington Labs. Callisto’s ability to track mutants leads Magneto to old friends and new soldiers when he frees Mystique, Juggernaut, and Multiple Man, and when Mystique is exposed to the mutant cure, Callisto takes her place as Magneto’s right-hand woman. But that role is still considerably smaller than Mystique’s, and the influx of new mutants means Callisto’s role is largely superficial in the film. She faces off against Storm at Jean Grey’s home and on Alcatraz Island (perhaps as a shout-out to the characters’ rivalry in the comics), but there’s no time for more development before she meets her doom with the rest of the Omegas by getting caught in Phoenix’s fury.
Played by: Dania Ramirez
Currently, Callisto is: Her death is undone by the events of X-Men: Days Of Future Past, but her current status is unknown after the timeline changes of that film.
Where will we see her next? There are no plans for Callisto to appear in any future X-films, but she could easily be introduced in the new XMU continuity with a version closer to the comic-book character.