User blog comment:IamJakuhoRaikoben/Chapter 366: 1000 Souls, Review./@comment-4449950-20140109212345/@comment-7418318-20140109223853

Ultear's father might have left because he was sure that nothing could possibly be done to save Ultear's life and he couldn't bear to witness his daughters death, so he ran away (Ur's partner did leave before she brought Ultear to the institude).

If he was desperate to escape this pain (and maybe phsychically unstable) he might have agreed to became a Demon in the hopes of this ending his pain... any maybe it even worked.

Damn... the more I think about it, the less sense anything makes. Not impossible, but still not making sense.

I hope there won't be any double connection between him Ultear and Gray, unless it's at least one generation apart (preferably more). Like him being Gray's grandfather and Ultear father or something like that.... it'd still explain the similar smell but without the complications that would arise from him being the father of both. Or maybe Gray's uncle... I guess that might work too, but not very well...

Wah... that's it for me. I'll just wait and see. Any more theories on this matter and my brain's going to tie itself into knots.