i've been sitting on this ask for two months now, letting it rack my brain haha. as you probably know I'm always reluctant to cast things in stone or say things as "word of god" for my stories. I'm still trying to figure a lot out about this new world and setting and story. How the characters intermingle and what their histories with each other are like and such, and I always get insecure that I'll say something is canon now and then change in down the line haha.
but at any rate- right now I'm really gelling with the idea that Rex's soul is spoken for, but not of his own doing. Something along the lines of his mother making a deal with a devil, promising that devil her first born, and that firstborn being Rex...
I also like the idea of that devil being Asmodeus, and part of Bub and Rex's mutual goal is to face Asmo and free Rex's soul from his clutches :)