Lego minifigs have "necks", a gap between the torso and the head. Perhaps adding something like that here could help!

You can also have the torso stick out further than the head, which would be good for buff characters.

Thanks for the feedback. I'll take a look at the heads again. I didn't intend for them to be flush, but u was having trouble getting them to look right. My inspiration was Legos figures, but they do look a bit odd flush like that.

I'll definitely have to clean up those stay pixels, thanks for pointing them out. 

You seem to have a few stray pixels around the head of the 3rd sprite in the top row.

This is an interesting style o: I think the top three characters would've looked better with heads that aren't flush with their torsos, like the bottom one; it would make for a more interesting silhouette.

You have some external grey AA on the characters, but with these angles and at this size, I don't think you need it. 45 degree angles don't need AA at all, and your other AAed areas should be just fine with only internal AA.