Umm ok maybe waaay off, but could Fitz in someway be Bob's son. Here me out, in GS Bob's "Human form" has gold-brown eyes, what if when Harry once let Bob out he ahh went along for a ride with someone, hey he apparently started an orgy once. Then if a baby resulted, would that kid be more prone to the spirit world, I don't know be able to hear it?
"He [Bob] looked . . . quite ordinary, really. Tall, but not outrageously so; slender without being rail thin. He had decent shoulders and looked sort of familiar. He was dressed like James Dean—jeans, a white shirt, a leather biker’s jacket." I wonder who Bob looks like, why he looks familiar. Or is it tied in with the following:
“There’s the Bob I know and love,” I said. “Love, ick,” he replied. “And I am and I’m not. I mean, you get that I change based on who possesses the skull, right?” “Sure,” I said. “So I’m a lot like I was with you, even though I’m with Butters, because he met me back then. First impression and whatnot, highly important.” Perhaps Bob's appearance is the amalgamation of past and present owners.