Although Daniel Ryves doesn't ever appear in the book, his presence is greatly felt. It's clear that he's the one who has instilled the importance of family and loyalty in both Alan and Nick, and by all accounts, he was a remarkable man. He would have to be to take in his ex and her demon child and offer them not only a place to live but a measure of security and love as well. Plus he makes the greatest sacrifice of all in giving his life for the lives of his family members.
Both Alan and Nick draw strength from the memory of their father, and even after Nick discovers that Daniel was not his bio-dad, we think Daniel's influence on Nick's life and the person (demon?) he becomes is evident, if in no other way than in the bond that has been established between Alan and Nick. We doubt that would have been possible if they'd had a father who treated them as anything less than brothers who both had an equal claim on his love.