She and JJ have worked together a few times before of course, mainly on Felicity and the third Mission Impossible movie.

No real world on who she's playing, but I'm guessing maybe she's a resistance character, perhaps filling the void left by Holdo and (probably) Leia (Although Poe can certainly function as a leader that way too). Or she could be a member of the first Order I suppose, or even some shady character like DJ or a CG character (Hopefully not the last one).