Nice~ definitely not what I expected from Sonic fanart tbh xD Really cool stuff :3
Main issue lies in values really. Everything is too dark; there's practically no distinction between character, background and the ground he stands on and because of that it's kind of hard to sell where the main light sources are coming from making those highlights kind of useless and shadows barely noticeable.
Their consistency is confusing too. It looks like the most of the lights are coming from the left side and behind becuz rim-light, but bit of the shadow I can make out under him one would think light source is in the front/above, so pay attention to that next time
more notes below op:
โข Colors: even though this is mostly monochromatic and gritty color palette adding more saturated colors could give character more powerful presence. I made colors more saturated on him overall and the rock/cliff? seemed too small for dynamic pose like that so I extended it.
โข As far as
perspective is concerned it would be more logical or... more balanced to have that leg to be more in the front and as such composition would have to be changed little too but I didn't have time to tweak that , but I hope even this much is helpful.
In conclusion, add more mid-tones, pay attention to light/shadow relationship and don't be afraid to add more "brighter"details to dark images. Good luck!