My theory is that she is a Vex Operant. She was created by the Vex to covertly gain information about the Light. Her weapon "has parts that shouldn't exist yet", not to mention that she can teleport in and out of time and space at her will. She isn't helping the Guardians, she's luring them into a trap. Did you notice the destruction of the Black Garden seemingly had no effect on the Vex Collective. It wasn't meant to do so, she simply is delaying the player so she can perfect her plan to stab him/ her right in the back. Also, did you ever notice that the Ghost and the Exo Stranger don't particularly like each other? But hey, that's just a theory. A GAME theory.