You have a light source coming from the top right, I see.
In that case, you should really accentuate shadows to her left side- really, try to create dimension. Think of her body almost as a construction of boxes, and where the shadow side of those boxes are.
For that reason, I think the left side (her right) of the face could have more of a constructed shadow. You can have a bit more shadow cast from her hair. And hair itself is going to have lighting and shadow too... right now, her hair colors are a bit all over the place... think about shadow side of her hair, and where upon her hair there may be sheen from light. To provide a little answer to that: Left side of her hair should be in shadow, as well as left side of any hair strands... hair which we see behind her neck should also be in shadow.
I'd maybe make the shadow cast from the chin to the neck, a lil' bigger, and don't forget how hair might cast a shadow too. Left cat ears has no form shadow to the left side ... think about how they might cast a shadow on her hair too.
Shadows on the arm are very thin, they could show thicker to give more form to the arms. Consider how a shadow might form to accentuate the form of joints as well -- particularly the inner elbow crease, for example.
You have a shadow just above the collar bone, which does not make any sense if light is coming from above. You are right that a shadow will be forming and cast her breast, but you make this continue all the way down her torso/waist, which doesn't make any sense.
