I found out from the original creator of the Roxas that this Sora is a recolor of that the little black piece is the hood of the organization coat. I didn't know what it was at first either so I just left it there in case it was important and that's why Sora's hood is painted onto the back of his 'shirt' instead of that piece being colored to be his hood. I'll probably get around to changing it eventually to match like it should. I just have had much time to do so lately.