I think my favorite thing about this piece is the variable boldness of the lineart. You don't see many people do that as much anymore, but it's definitely a choice I like; it gives it a good comic feel. Your palette choice is good. I can't tell if the leggings and sleves are pure white, but they blend nicely with your background color.
One thing that I could critique, though, is some of your shadows and highlights, especially on the hair. You have the thick anime style highlight going through her bangs, but then you make the shadow on the left stop at the highlight. If it were a real picture, anywhere that's in shadow wouldn't get the light that's creating the highlight, so it's weird to see the highlight in a place where light isn't hitting the subject. Also, there are some places on the ponytail that feel too well lit; like her body isn't casting a shadow. I guess the hair could just be really far back from her body, but the perspective suggests otherwise.
Anyway, not bad at all. Just think about that shadowing thing, and your pictures will turn out even better.
The background is kept simple because it was intended for a T-Shirt.
Shading is something that I need to work on.
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