This is really nice. I myself specialize in sketching pre-existing images or photos. If you care for it I have a few tips, otherwise just ignore the following . Firstly I' just like to point out how good your proportions all are, well done with that! Now If you really want your drawing to stand out, pay attention to the tones of the original. Try and replicate the contrast in your sketch, keeping the light parts light and dark parts dark. DON'T BE AFRAID TO PUSH HARD ON THE PENCIL TO GET DARKER TONES; I think this is something a lot of people are afraid of. You have the shading all in the right place, perhaps next time try really darkening those shadows to bring out the image. Maybe experiment with softer/darker pencils to get those darker tones with a softer feel. Overall I do love it, and I'm going to watch you to see how you progress.
I have done a sketch of this Yuna image so feel free to check it out to see how I go about the shading. [link]