I'm gonna be a little nit-picky, because overall I really like the piece
He seems a bit too centered left to right. Then again, this is for a mobile card game, so perhaps there's a reason I don't know about that you wanted him where he is; however my mind keeps wanting him to be shifted to the right to bring his eyes to that 1/3, and the weapon moved out away from him just a tad to get close to that other 1/3 (and more contrast in that metal as well! Weapons often want the attention.)
Take this with a grain of salt, I'm no professional yet. Just throwing in my two cents. Also that's why I didn't put it under "critiques."