A few things I spotted.
* You antialiased onto the background. Try tweaking the background color to something much darker and the crunchy pixels immediately show up.
* Upscaling the image is EVIL ^_^ as it makes edits harder. Leave the upscaling to the script built into the board, please.
* Right from the start, you seem to have given RD a plushy-like modelling. This is weird for an athlete, and also makes it rather difficult to do crisp, readable shading.
* You seem to use only 1-pixel runs of AA... If this is in an attempt to reduce banding, it's largely unnecessary. In this situation of toon-ish lighting, antialiasing rarely causes banding, because of the simplicity of the shapes involved.
* There are some style questions. Exactly how realistic are you trying to be? What line weight are you aiming for? These factors are currently inconsistent throughout the picture. Addressing these questions is something that is much easier with fewer colors, BTW -- there are many many reasons to keep colorcount low.
* Which brings me to the question of colors. You use a lot (50). However I don't really see them, much. The appearance of your sprite is more along the lines of 36 colors, meaning you're probably wasting about 14 of them.
Here's an edit partially addressing these issues:
Overall, I feel that your 'picture idea' is satisfying, while the expression of that idea is unsatisfying. I'd say that this indicates the problem is artistic in general, not specifically to do with pixel art... so I'd recommend doing some traditional art... experimenting with all those things like line weight, lighting, etc that you can quickly do with just a pencil.