This concept is pretty good and fun.
I love your lines, they are as clean as they can be and have excellent weight.
I don't think the airbrush your using for shadows and highlights is helping your work. It clashes too much with the otherwise "sharp" style, which is your strong suit.
You could experiment with cel shading or even halftones, that could work best for you.
Anyway, nice work, looking forward to your future characters!