Major Dark Phoenix spoilers ahead. While the cast of Dark Phoenix consists primarily of actors who were already part of the X-Men franchise, there is one major new cast member this time around. In Dark Phoenix, Jessica Chastain plays Vuk, a character who joins in on the battle to rid Jean Grey (Sophie Turner) of her extreme powers — just not for the same reason as everyone else.
Vuk is a D'Bari, which, in the comics, is an alien race whose planet was destroyed when "the Phoenix consumed their sun", according to the Marvel Database. In the comics, D'Bari are green when in their alien form, but can take on human forms, too. While Vuk took on a male form in the comics, that's switched up for the film.
Clearly, Chastain is Vuk in its human form, and, according to SuperHeroHype, Chastain is playing a regular human before Vuk takes her over. Vuk is interested in Jean Grey, because of Jean's powers and wants to drain them for the D'Bari to use, according to IndieWire's review. Variety's review explains, that Vuk "tries to enlist the alienated Jean in helping her to take over the planet" and that Vuk appeals to Jean by explaining that people don't like how much power she has because she's a woman. Digital Spy writes that Vuk tells the remaining D'Bari survivors, "We can resurrect our race. Begin again, here." Put simply, the D'Bari planet was destroyed because of the Phoenix and now they want to use the Phoenix's power to take over Earth for themselves.
The reviews might be out now (the movie premieres June 7), but the identity of Chastain's character had long been, officially, kept a secret. Chastain did give some hints (well, a little more than hints, because she's bad at keeping spoilers quiet) leading up to the release.
On The Graham Norton Show, Chastain said, "I, for some reason, was told I'm not allowed to say that I play an alien, but I basically play an alien." She added, "I come to Earth to try to take this Dark Phoenix power for myself and my own species, as one would. And in doing so, I unintentionally empower her."
She then explained one of her inspirations for how she played the role. "I am a shape shifter, so I am supposed to look like an elevated form of a human being. The one person I think of that way is Tilda Swinton," she said on the show, according to Digital Spy. "I'm not convinced she's a human being. I think she is ruling over some planet somewhere like the goddess she is." Well, the white hair does help with that connection.
Chastain also spoke about her character with Screen Rant and said of collaborating with director Simon Kinberg, "For me, I mean, the look of the character was something that I brought to Simon and we worked together to hone in. And then things about the clinical nature of her, this idea that she wouldn't be very emotional because when I read the script I was like, it's a very emotional script, so I think it's nice to have a character who sees that as weakness and is giving another kind of performance for that to go off of."
Kinberg talked about Chastain's role with Screen Rant, as well, and said that he pulled inspiration from different characters in forming her. "I really looked at all of the different characters in the Dark Phoenix Saga that Chris [Claremont] created. I looked at the Dark Phoenix story in the cartoons, because for a lot of people, that's the way that they first experienced and primarily experienced the X-Men before the movies was the cartoons," he explained. "...I essentially created a villain character that had elements, let's just say for lack of a better description, elements from the stars and elements from Earth."
And thanks to Chastain, those elements include Tilda Swinton, too.