Responses
I think it might be because he both respects her a great deal, and because he interacts with her directly the most. Tbh it could also be a bit of fear and/or caution since he has directly seen her destroy a planet and kill another god with barely even a wave of her hand. Depending on how things go I do slightly get the feeling there's a chance he might call the magic god by his title too. I've seen him call the gods by their titles when face to face, but you're right in that he does only call the demon god by her title no matter the place.
The only God that he refers to by their title is the Demon God. He calls every other god, that he knows the name of, by their name. He refers to the Magic God by their title but he doesn’t know their name yet since he hasn’t found their altar yet; but he has found the Demon gods altar, so we know their name is Lucifer but he still calls them the demon God and that’s the only one he does this with