Damn that neck I can't tell if Sakuya is calm but mouthless or if she is freaking out

Krugger said:
Damn that neck I can't tell if Sakuya is calm but mouthless or if she is freaking out

Considering the shape, I'd say that's just a really low mouth.

It is. See the original lineart (the parent post) for confirmation; the mouth is shaded there too, and there isn't any background shadowing in that version.