User blog comment:PUNCHURPREGNANT/Gray and Silver Predictions/@comment-24066070-20131113101844

IF Silver is anyone's father he's Ultear's. I'll give some reasonings behind this claim, feel free to agree or disgree:

1) IF Silver was ever human, he would have been really strong as a human, otherwise he won't be "selected" for the demonification process and he certainly wouldn't become a Demon Gate. Now remember Ultear being born with "too much magical power", perhaps she "inherited" this from her father.

2) It has been confirmed that God Slayer magic can be learned from a book, as Chelia did. Therefore it is likely that the same applies to Demon Slayer magic. Possibly Silver was already a Demon Slayer when he met Ul and inspired her to learn Ice-based magic as well. The one true Demon Slaying Spell she was able to master was Iced Shell, hence it being the only spell that worked on Deliora and even kiled him.

3) Ul and Lyon were visiting a place attacked by Deliora when they found Gray. It could be they were just passing by, but perhaps Ul might have been looking for Silver the Demon Slayer. She might have thought that perhaps Silver would be hunting Deliora down and that he would look for clues near the places attacked by the demon. I know this argument is far-fetched, but exploring this counters the theory that Silver is Gray's dad, since Ul or Gray didn't find other survivors in the ruins + Gray would not have left the town had there been a chance of his parents surviving.

4) Silver was seen visiting a grave. The impression I get is that Ultear died a natural death at the end of chapter 339, so perhaps he made a grave for his daughter as she was the only family member to leave a body behind (Ul became ice and melted)

I know this theory has some possible flaws, as I'm not sure Ultear died and I'm not sure whether Iced Shell is a Demon Slayer spell or not. However, going by the reasons I mentioned it just seems more likely for Silver to be Ultear's dad. Besides, if he is Ultear's dad the author wouldn't have to "revive" Gray's dad, who is a dead person (like what happened with Lisanna, confirmed dead for many chapters and then suddenly turning out to be alive).