I will begin with some technical things and then get on to a more conceptual critique, if you can bear it.
Colourwise you have a very dark brown and white which you use only in the eyes. Why not use these colours in the rest of the sprite? The white can be used for extra highlights, like a bright specular shine, and the eyebrow brown (eyebrown) can by used as your darkest colour to make those outlines pop out a bit more.
Secondly: you have a good idea using some dark lines in with the light ones to make things stand out (like the hands from the sides of the body in the frontwards frame) but you could go much further with this. Use your colours to indicate volume! Unless you want to have it look like a cookie at this scale, this is the way to go. The other thing is that your outlines are so light, and so uniformly coloured, that they come across more as a kind of glow, a slight falloff towards the edges, than as an edge. They won't add much to make the sprite distinct. If you are going to go that route you may as well drop the outlines entirely and just do flat colours or areas of light and shadow.
Thirdly: you have rendered his butt in exactly the same way as his crotch and this makes me uncomfortable.
Now is where I get conceptual, and I urge you not to take this as me singling you out. We get a lot of muscle baby characters like this here: squat little naked RPG men, in the style of Final Fantasy or RPG maker or basically any archetypal Japanese RPG. It's to the point where if you see an "rpg character" thread you pretty much know to expect exactly this or the clothed, probably blue/green haired equivalent.
Now people may have been making game art for a few decades but I don't think that means that we've hit on the best solutions for everything and emulating other videogames is not at all the ideal solution to making game art. I'm not accusing you of copying, or saying that cliche should be avoided above all else, but this doesn't strike me as the kind of solution you'd come up with on your own to "I want to represent a person in a 16x32 pixel box." I encourage you to learn from other art, but to use it in making your own solutions and just really thinking about what you're trying to show and how you can best represent it.
I just made some quick drawings of bodies at the same scale, using your colours to hopefully show you what I mean:

I'm not saying that these are the best solutions, or even better than yours, but there's a lot of possibility even within this narrow idea that can be explored. One other thing is that I draw pretty volumetrically which is something I just prefer. I'm considering here the effect of an above the head light source.
Also note that Joseph's critique of using upward tilted perspective applies to me as well
