Scumspawn: All the other demons serve you out of fear and terror, but I've served you and sacrificed myself to your will because I love you.
Satan: "I beg your pardon?
Scumspawn: I love you. I don't fancy you. No, that would be physical and weird and probably quite dangerous. No, I love you in the widest possible emotional sense.
There are hints that it might not, in fact, be entirely one-sided. "Scumspawn looks good in that studded collar, doesn't he?" comes to mind.
Note that the few times Thomas shows his soft side it is mostly (if not always) doing something nice for Scumspawn.
Gary seems ever so jealous about the amount of time Satan spends with the Prof. Satan, in turn, is oddly protective of his favourite mortal, outraged at Thomas strangling him and stopping Nero from crucifying him. It's almost as if he cares...
Magnificent Bastard: Satan manages to avert a demon rebellion by pointing out that if they rushed him at once, about half of them would survive. The second half. Naturally, none of the striking demons are eager to be first, and Satan manages to stop the rebellion and put the check-in demons back to work in one stroke.