Here’s a tweaked version:

I think the three main takeaways here are the direction of the light, the volume of the character, and HSV shifting.
Light should always (usually) shine from a specific direction. In your case, it seems to be somewhere off in front of the character, to the right of the canvas; however the shoulders and hair contradict this. If you shade under the arms and the highlight the right side of the hair instead of the left, the sprite looks more consistent so it’s easier to tell what’s going on.
When you shade a character, you’re literally telling your audience “This is the bit that’s in shadow”. If your shading is only one pixel wide (on a big sprite, that is), that suggests that your character is paper-thin, which probably isn’t actually the case. If you widen the shaded area, it gives a better idea of the character’s volume.
Finally, HSV shifting sounds like a crazy technical thing, but it really isn’t. All it means is taking the color of light into account. During the day, the sun shines yellow, so you want your highlights to be yellow-ish, and your shadows to have less yellow (or more blue) in them. Mortmort has a great video on choosing colors that I recommend here:
https://www.youtube.com/watch?v=QhgSM_tnPM4A few misc things:
I messed with the positioning of the right arm. I think it’s personal preference which you think is better, just thought I’d put it out there.
Always exaggerate colors. They tend to look over-the-top close-up (the skin tones certainly did), but when you zoom out they’ll fit together better. If it still looks bad when you zoom out, that’s when you tone them down.
I continued the pattern on the clothes into the shadowed area. Again, personal preference.
I only HSV shifted the skin and shirt. The hair, pants etc haven’t been done. Then again, the pants are already blue so ¯\_(ツ)_/¯ And of course you have the other sprites.
I noticed an extra shade of blue on the pants, which could be easily replaced by the other one.
I noticed that the pattern on the left and right side of the shirt is uneven, so I went ahead and mended that.
Hope I helped! I know I had a lot of criticism, but I really do think they look great, and I especially love the question mark tee on the little guy! They just need a little refining is all.