Right now, the biggest problem is your pallet. The sprite has around 11 colors. Not bad really, but some of the colors are just so similar, that some could just stand to be replaced, or have their color changed slightly. I took a shot at it. The new one has around 8 colors.

It looks very much the same, but has quite a few less colors. I don't know if you were aiming for color conservation, but it's a good thing to keep in mind.
I also added some highlights to the hair and eyes. When working with brown hair, a shade of skin color quite often works very well for highlights too. There are also a few other things, but I'm running really short on time, so I'll let the edit speak for itself. Hope this helps!