Your main issue, from what I see, is generally a lack of proportion through the body.
It's most prominent in the arms, waist, shoulders, and neck.
Legs: Are way too short when compared to the arms. Generally the arms and the legs should be about equal in length with the legs being a bit longer. I can see why you made this mistake though with how you drew the waist.
Waist: Is too way too low. With the way you drew the clothing, it appears as though the legs begin around halfway through the skirt (which they do on Envy's character). If you look really closely at it, she has munchkin legs, which look very offputting when you compare it to her upper body.
Shoulders+Neck: The shoulders are a bit too big. Envy is a character who is very physically fit, but she doesn't share a physique with Captain America (Captain Dorito). It's not anything excessive though. The neck is also very very thick. This could be a style choice on your part and that'd be fine, but when I look at myself in the mirror and compare my shoulders to my neck, I see that between my shoulders I can fit 3.5 neck sizes between my shoulders. Envy here can seem to fit 2.5.
Everything else I see is just stuff you'll pick up from general practice and knowledge of the software. If you're not using Paint Tool SAI and would like to shoot me a PM and I'll get you hooked up. If you are, I would recommend looking into the stabilizer setting to make your lines look better overall.
It's also never too early to experiment with shading and highlights. Your art is already at a point where it would improve it's quality and give you much needed practice.
Hope I helped.
