I feel like this piece leaves quite a bit to be desired. Most moticibly, you forgot the white fur on Tails 's chest, whitch is quite a big detail. Also, the hair atop his head looks really weird, it seemes to be facing sideways while the rest of the character faces forward. It's also really odd that it's the only part without outlines.
One thing I will give you is that the face looks quite good.
The tail overlaps with the hand in a very weird way, the line between the white and yellow part of his hand just feels way too similar to the tail's outline.
Additionally, the ground and background are very plain, it's just straight up green rectangle and cyan rectangle. Not even clouds, sun or a dirt layer.
Lastly, while not nessecary and maybe difficult, I think some shading would be nice to have here.