Take this with grain of salt but hearing that Gwendoline Christie is playing a stormtrooper who defected from the Empire. Not posting on the site but it comes from an interesting source.
Now, if the releasers aren’t taking the rumor too seriously, I’m not. While Christie can be really masculine in Game of Thrones, I can see how the rumor takes shape, accurate or not. That said, in Star Wars, anyone can play anything from a Queen to a Wookiee.
My bet? Perhaps she’s playing one of the hired mercenaries we’ve heard about.