First off, I want to say this is a very beautifully rendered piece. The limited color pallette of grey with the splashes of color from her hair, belt, and his veins look very nice together. This isn't a busy piece, it has a nice flow. I like your somewhat low-key use of dramatic light set-up from the bottom aimed up. It's not too harsh, or too light. The texture on his head is very interesting. It looks like water refraction swimming on his forhead.
The only suggestion I would make is his hand. He has like a squid or an octopus looking head, then the horns obviously. The bottom half is still humanish, but it looks as a lizard or reptile would look. Also his barbed tongue. He is unmistakably humanoid, but he is monstrous. I think his hand should be more in tune with the wonderful work you did on his head. I see only three fingers, but it could go farther.
All in all, very very nice work. I love your stuff!