I thought for a while now as to how guardians were chosen, and I think I have an idea... And it starts with Robert Daly. The DNA coder used by him in the episode of "Black Mirror" allows for him to create endless digital clones of this subordinates, just like ghost. All beings chosen by the traveler had their DNA taken in some way as to be able to have them be created endlessly. The ghost acts as a USB that holds their information to the trans-matted wherever in the universe.