It's not bad but there is room for improvement. I do manga art as a side job (small commissions and such.), but I studied a lot of realism to help with my proportions.
The main error here is anatomy. For example, her right arm is much too skinny. Wrists are usually the thinnest part of the arm and then the arm gets thicker from there. The way the arm is drawn here is very straight. Her right shoulder also seems "off" in a way. For one, I think it needs to be a bit thicker.
The way you drew the facial features is really nice, however, their alignment is off. I flipped the canvas to help see errors and drew the lines that the face followed here: (http://) prntscr. com/awfpim These lines should be nearly parallel, but there's quite a bit of variation. I also drew a line where the middle of the face should fall, considering the nose and forehead. The middle of the chin doesn't fall on this line, though. Everything should nearly line up.
The shape and bone definition on the head are very nice though. The head and body and proportionate to each other. You also got the hand right (hands are about the same size as the face).
There's still a lot of time for you to practice. I can see you becoming a pretty good artist if you keep working at it.