[@The Roman07]

That is a pretty sweet character. I want to have it work in the Blood Compact world, so I'm going to give my suggestions on how such a character could come to be.

So, in Blood Compact, there are no demons nor angels, nor loose spirits, as far as anybody knows. Nobody knows the true origin story of werewolves and vampires or if there's really a God or what. But there is psychic shit and mind-control and individual consciousness, call it a "soul" if you will. So...

What if we say that Rin is, or was, a vampire, and she had the unique ability to "possess" others? I would explain her "possession" as a kind of psychic attack, or mind-control, except that she actually transfers her own consciousness into her target. Perhaps Rin abandoned her own original vampire body long ago as she lept from victim to victim, until one day, she entered Rome. Now for some reason, she wasn't able to kill Rome, but she did find a host that could sustain her. Maybe Rome was already a strong vampire thrall, or her thrall, or a thrall of one of her descendants, or maybe Rome simply had the right genetics. It doesn't really matter. The two stuck together after that. 

Rome has abilities that would make him a good fighter for Soldier, such as Rin's ability to sense other vampires and possibly psychically attack them. However, Rin's need to possess and actually kill humans to survive is a big problem. I think it would make for some good drama for your character. Soldier is not going to want to employ an agent they have to feed with lives. They might just target Rome for execution due to his lamentable feeding habits. It's up to you how you want to play that out. 

The Gabriellan vampires tend to have psyonics, so perhaps Rin was an old member of their bloodline. 

What do you think of that?