the character at te back has a very well stuctured pose and actually seems a little like they are walking. the lighting on the hair of the character at the back is a nice touch and gives that character some realism in the way the hair shines. the evil eyes in the background work well with the characters and look interesting to the viewer, finally the line art is well done and keeps a simple looking style which is effective all around.
The shading needs to be alot darker on the character at the front because on theface it almost looks as if there is no shading at all. The shading on the character at the back is good if your aiming for a leather effect, but to get a more realistic look I reccommend making the shadows smoother. The shadow under the character at the back needs to be much softer around the edge, as I assume this picture is at night I would have thought theres no direct sunlight.
However the thing that bugs me most is the fact that there is no shading on the hair of the character at the back, this part needs shading to keep the flow of the picture constant rather than jittery.
Finally the evil eyes in the bottom left corner of the picture, they seem somehow flat so maybe remove them and the image would look better.
How to improve:
I reccommend that you look over youre shading technique and try to keep it covering the character with smothe edges accasionally and sharp edges in places to show different distances between the object and its shadow. also take in to account which direction the light is coming from and what time of day it actually is.
all of the above are just pointer and what I think, it is up to you to take them onboared and improve your style. maybe change what you draw, try humans or dragons or vehecles or something different from sonic characters, and with that this critique is over...