I don't know if this has been mentioned before, but anyways. I noticed something when they were fighting the fallen miniboss during the full demo. When the hunter dies, an orange globe hovers above the spot where he died, and that's what the warlock used to "resurrect" him. Now, if you look closely you can see that it bears a striking resemblance to the "ghost" only this time it's orange instead of blue. So maybe, when the game starts, as they said, each guardian gets their own ghost to help them reactivate old technology, but I'm also thinking that it might store your soul in case you die.