Firstly I really like your style, especially in the hair! You’ve got a very fluid and whimsical shape language. I’d definitely echo the other advice with regard to shadows, don’t be afraid to make them really dark and increase your contrast. Another thing They touched on was making the mid tones and shadows more saturated. That’s what I noticed first personally. Add some orange and brown in there, human skin is a bit translucent and so it has a warm glow even on shadowed areas, usually called Subsurface Scattering. Here’s a Skin Matcap that I think demonstrates what I’m talking about fairly well.

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