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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With all likeliness I think she a guardian for the future that arrived the same way the vex did. She comes from a time where the darkness wins against the traveler and the guardians and for some reason our guardians are of great important in defeating the darkness. Also this is just my thought but the vex are machines that the darkness created in the future to stop the guardians from succeeding to defeat the the darkness.