The only thing that he been established is that her character will take on a similar role to the one inhabited by Rebecca Ferguson in Rogue Nation.
Exact details regarding how Vanessa Kirby’s character will factor into the overarching narrative of Mission: Impossible 6 remains fairly unclear as of right now.
This makes sense, as we really don’t know anything about the plot of the movie yet.
This could mean that she will be an antagonistic agent, or it could mean that she is an ally for Ethan Hunt, or it could be anything in between.