Her character, Wolfsbane, is a girl who transforms into a wolf. Also starring is Anya Taylor-Joy, who first became famous for The Witch and then went on to star in Morgan and Split. She told Refinery last year that she was excited to play a variety of roles rather than get put in a box, and it looks like she's continuing that mission. She'll portray Magik, the sister of Deadpool's Colossus whose powers include sorcery and teleporting.
Magik and Wolfsbane are part of a group of teenagers who embark on various adventures in the film. We don't know who will play them yet, but Fox is trying to avoid another Hollywood whitewashing situation by casting a Native American actor as the character Danielle Moonstar and a South American actor as the Brazilian character Sunspot.
Josh Boone, director of The Fault In Our Stars, will be directing New Mutants. "We’re so excited to explore this new part of the X-Men universe," producer Simon Kinberg told Deadline, "and so excited to do it with Josh, who is uniquely suited to tell this story about young characters."
Williams follows in her Game of Thrones co-star Sophie Turner's footsteps. Turner is starring in another X-Men spinoff, Dark Phoenix. Her character, Jean Grey, can read people's thoughts and make things move with her mind. "She can't control her powers unless she either fully stops them or lets them all loose, and that's kind of Phoenix," she told Entertainment Weekly. "I think in future moves to come we'll definitely see a bit more of that."
New Mutants is set to come out on April 13, 2018, followed by Deadpool 2 on June 1 and Dark Phoenix on November 2.