Something interesting that I've noticed no one seems to have very concrete ideas on. Is where does the Mysterious Exo come from? Now, I am making a bit of an intuitive leap here, so I may be completely wrong. She (if she is the Exo speaking to us from the trailer we saw), is a Guardian. But not of the Last City. But of another, lesser known area. In Norse Myth, the Einherjar are the chosen fallen warriors who assist the Aesir and Vanir during Ragnarok. These Eiherjar are sent to one of two places, either to Valhalla, Odin's Citadel. Or to Folkvangr (field of the host/army), Freya's domain. So, here's my theory. The Exo Stranger is Destiny's equivalent of Folkvangr, while we're from The Last City/Tower, which is the equivalent of Valhalla. Our ghosts are our Valkyries, reviving and sustaining us. What do you guys think?