She's a mystery, an enigma. It excites you.

[ Castiel wasn't dumb, he knew the way that Lucifer worked. Immune could either mean something terrible for this woman, or something exciting for Lucifer, and he was relieved to hear that it was the latter.

Relieved, for some woman that he had never met. ]

She makes you mortal? I didn't think that would be the kind of puzzle that you found pleasure in.