[ It's so very tempting to be cruel and mock her naivete and trust, but Sebastian has lived as long as he had by reigning in his nature where appropriate. There's the visceral instinct to proverbially bear his fangs, but instead, he just hides them behind the polite smile as he always does. ]

...You were very close when you asked about the mark of the contract that both the young master and I bear. I very nearly told you then, but out of consideration, you stopped me. I do not point this out to mock you, but instead, to thank you. I hide what I am out of necessity, not out of pure malice.

[ Partial malice, certainly. But for the most part it truly was necessity, since it would be quite difficult for him to do his work if his existence was obvious, now wouldn't it?

The saltpeter is ground down into a fine powder, so he pauses to gently set it on her workbench. He leans down to do it, and it's to catch her gaze. His expression is one of bashfulness, almost reluctance, but that is the most deceptive thing about this. Sebastian can't lie, as per his contract. But a false expression wasn't a lie. ]

I am a demon, Master Sullivan.