Your designs are looking good, but you have an awful lot of pillow shading going on. Let me just create a number scheme for these guys:
1234
5678
In sprite 1, his stomach plating is pillow shaded, but as he is metal-plated it mostly works out fine.
In sprites 2,3,4,5,7, and 8, the cloth needs some
folds. It can't just be pillow shaded, or they look at best like the character is wearing a skin tight body suit. In 8, it makes her dress look like plastic.
The hair in 4 looks very wacky. What is going on with that bit sticking out? The shape and shading do mot make it clear.
Sprites 5 and 6 are some of the more pillow-shaded sprites I have ever seen. They need a
lot more work into texturing and defining the shape of their parts, and the materials. If nothing else it will give them some visual interest. It will also define what they are made out of, which is very unclear at the moment.
Sprite 7's hair is, again, very very pillow shaded. It has absolutely no character, it is just a blob of red. There are no strands or shapes in it.
You're off to a great start but these need more detail and work to be particularly high quality. As it is these would be acceptable though. Well, except that they are only 1 frame