Hey! Your concepts look very nice and clean!
I like how you stay in one lane with you choice of colors!
The faces of these characters, especially the third one, look very good!
The designs of the clothing are thought through and easy to read. The materials are well rendered and recognizable.
And the anatomy of the body and the hands are very well executed.
I noticed though, that some cast shadows are missing, especially in the chest areas of your characters. You included some cast shadows on some parts, but not everywhere. Some of the belts on your first, third and forth character, aren't casting shadows on the layers below them. It looks like these parts aren't stacking on top of each other.
And try to plant the feet of these characters of the ground and adding shadows below them. And completely white backgrounds can be a bit too bright and distracting.
Otherwise very nice character concepts! I hope my feedback is helpful!
