I agree with HarveyDentMustDie. He looks squished from the side, and it's not just a matter of cartoony style vs realism. It almost looks like you tried to do the body from a top-side diagonal perspective, and then did the head from a simple side perspective. I think it's partly because he's got very long legs, and yet he's only a few inches from the ground. This makes his legs look very long when he's stretching them forward, and very short when they pass underneath his body, before they're stretched out again in the back.
And while the skull looks pretty cool in your concept design, it just looks very odd in the game sprite. The cheekbone bears no resemblance to a real cheekbone, and it looks like his eyesockets have been extended across his forehead, unlike the concept sketch. Your concept has a nose, your game sprite simply has a flat face with a hole in it. And it's not actually a hole, it's a black surface. (If it was hole, his profile would have a gap where the nose is, not a vertical line.
I would do a lot more work on the design of the sprite before working on the animation, otherwise you could eventually be irritated with these flaws in time and realize that you've put a lot of work into a design you're not happy with.
Otherwise, a very interesting and original project. As a half Venezuelan, I'm looking forward to seeing your progress.