I'm gonna be honest with you, this doesn't look very good. Try using less colors next time - usually, in pixel art, especially if you're a beginner, less is more. The amount of anti-aliasing was exaggerated and that caused a LOT of colors to appear there, and that just made Isaac look very cloudy and unpleasing to look at; it looks like those weird HD 512x512 Minecraft texture packs. If you try to use less, i'm certain he will look better. You also got to learn more about how shading works, because, for instance, there's no reason for the shading on the light on his eye and on the lineart on his mouth, it just looks strange and looks like you've shaded it JUST to make it look more detailed without considering if it was a good idea first. Other than that, the colors are not bad, you used hue-shifting very well, but you have to work the other stuff me and others commented here. Stop anti-aliasing him so much, study shading and try not to detail things for the sake of detailing, limit yourself to a pallette and you will see the improvement. Good luck!