Can't say I'm a big fan of the backpack color either.
Also, his hands seem to have strange dark and rough shading, unless they're gloves in which case the color is too similar to hands, haha..

In some of the frames, his hair ends quite abruptly near his neck, which makes it look like a hairball coming out of a green pipe..
The white line running through his sleeves sorta looks like a glossy highlight due to the bright green color of the sweater.
Kinda nitpicking here, but once you notice things like this it tends to stick..
Very charming and well done though, will be interesting to see it in action!