That's a nice little sprite and animation. If there's one thing I'm really useless at, it's characters and character animation, however a couple of things instantly stood out at first glance.
Firstly, the legs. I'm not sure what the segmentation is supposed to be - whether it is wearing matching colour knee-pads or if its some kind of bandage-wrap, however on the right leg (his left) the bottom line seems to get left behind in the leg up/down motion. I feel the line needs to be altered (not just raised) for the upper position.
Secondly, the arms. The simple up down motion seems quite stiff and as if you've just moved the 'arms' layer of your sprite up and down. A little variation in the bend of the arm, or raising the rearmost hand an extra pixel would help offset the stiffness.
To double check myself and see if what I was trying to say worked I started a quick edit of it. However, after attempting to tweak the knee and arm, I noticed that some of the shading didn't seem very consistent. In particular the black outlines, so I moved these to fit better with your apparent lightsource given the majority of the shading. I also tested out using the white to add some extra highlight to the legs, though perhaps it is too much. I haven't altered the palette in any way, though I feel many of the shades are too similar and it would benefit from some palette tweaking.
Anyway.. this is only a quick edit and I'm really no good with characters so perhaps take my edit with a pinch of salt, but perhaps it will highlight a few things that could use attention, and I know there are many here who could produce a much finer edit than this one.
-Z-
P.S. Welcome to Pixelation
