Oh and I just realized something, we still have no idea on what that Lost Soul thing is. If he is what I think he is, im willing to bet he has something to do with Trevor's transformation into Alucard (if that turned out to be the case). I mean he is called the Lost Soul, and what does anything called a Lost Soul want? A vessel.
So in essence once Trevor presumably fails, the Lost Soul who is on speaking terms or appears before Trevor as the Demo evidents would imply that once Trevor dies the Lost Soul takes this opportunity to take Trevor's body as it's own, either preserving both consciousness into one or simply overwriting the old one with the Lost Soul's mind, or possibly a body swap if you will.
Trevor's body>Lost Soul's body and vice versa.
Furthermore with this transfer a transformation of power is initiated, causing this vessel to obtain demonic power.
Another way is that the lost soul gives Trevor a second chance, but at the price of a majority of his human memories and basically merges into one being.
Who can say for sure though?