The face is drawn in 2D (front and side). And in my 3D software, I use the drawings of the face as a reference to modelize the head.

The head is a mid poly mesh. The hair, eyes, ears and mouth are planes with a transparent PNG texture. It means , when you turn around the head, you see the trick.

Note: The skin, shadows and black hair, are painted in Blender. I used a 2$ addon called "PSD Layers" to separate my colors onto different layers. The outline is the only generated effect.

Thank you for reading!