Hey! So weird, I was going to post a similar topic for the same reason... I'd like to build a portfolio too. I'm sure we can critique/keep each other motivated, yea?
Onto your sprites. I like your colors, character designs. In general at this point I think the biggest improvements could be gotten from 1) eliminating unnecessary banding and 2) enhancing the focus/readability of each sprite. For example, here is an edit to your steam knight:
His thumb was not readable. I fixed all the banding I could find, and cleaned up the shield some. Too many accessory details detract from the sprite as a whole. I separated the shield from its body with darker tones, as well as his chest/left arm.
The green character is slightly over-AA'ed, and the other two could use some banding elimination, perhaps similar contrast as the knight too.
