After a few days of attempting to push away the face shadow, I found a quite easy way to achieve a more 2D like effect. I modified the skin material shading mode to subsurface and added an extra direction light which will rotate according to the position of player camera. Since my project is a VR project so this method is quite handy.
I am now trying to understand the reason causing this shading difference, maybe is due to the extra light reflection under subsurface shading mode?
Followings are the pictures comparing the results.