I'm gonna have to say Guns N' Roses' Sweet Child O' Mine. At least for my main, Genesis.
Genesis and Shilo are the two main characters of the comic. Genesis, being a foster child who was adopted by Shilo, is often sung this song when she is feeling down on her luck. I find it especially endearing seeing as how Shilo, being a fuzzy little demon whose sole purpose in life is to protect children, is very scared and clueless to be put in such a predicament but is trying his best. Though the song was originally written for a love interest, I still feel that the lyrics apply in a sweet and sentimental way to the two characters.