When we first saw this mysterious guardian in the very first mission I though that she would help us later on. However, later on during the first moon mission we see her face and because of the cloak I'm guessing she was a hunter. In the scene that we see her she seems more like a ominous threat. Why else would she let us to fend off the hive horde alone.
My theory is she used to be a guardian like us but then her ghost died, and she went rogue trying to steal other guardians light, and become the traveler's worst enemy yet.
I don't know but I wanna know. any thoughts, theories.
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She seems to know things we don't and the discription of the gun she gives you says something about the technology in the weapon shouldn't yet exist. So my guess would be she's from the future.