I have songs that I think would be absolutely perfect for Spuffy, Spaith, Twillow, Xanya, oh, and I have a real dandy that describes Bangel perfectly. But having read the entries that everyone else has posted, and having no idea what a single one of them is, I think that I'll just stick with my own Broadway show tunes and classic C&W songs and let you all wonder.
(One that I will share with you, though, and feel free to laugh if you want to, assuming that you can even find it anywhere, is the song that I would pick for Spike and Faith. "Two Lost Souls" from the Broadway musical and movie, "Damn Yankees." Now, go ahead, laugh at the old man all you want to)