Could you be more specific about what looks out of place, and what looks to have lesser quality? That'd help a lot when we go through and refine them.
Sure thing, I would say the best one out of the bunch is the fish guy. Had some fairly good shading, very appealing colors and was well thought out I'm sure. Then you take a look at the furry or rock guy and it just doesn't seem to match. All the characters have the same eyes, so if you're going for the pure white look on one, why not do it for the others? One having black beady eyes and the other the white ones just doesn't mash that well to me.
The
furry is the closest style to the fish guy, but it suffers from poor texture in my opinion. Maybe I can find time to do an edit with any ideas but try for a bit more of a larger chunks of fur shading approach. You tried a similar lit outline approach but it doesn't fad so much as just the legs are really dark and contrasty and the rest is not. Try thinking about where the light hits it and lighten it up there. Also about the furry, the colors just don't quite seem up to par with the fish. Try shifting your shadows to blue and maybe work on your main base color too. It's a bit bland. Just noticed his hair is identical in both, that's a bit cheating no? Try adjusting it so ones the front and one's the back. Shouldn't take long but it'll do a lot for you.
The
rock guy again with the eyes not matching the others and the outline thing mentioned in the last one. Outline seems more broken in this one and shading doesn't seem very thought out. No mouth and no moss/hair. Colors, I'm not so convinced you're doing the most with your colors here. Try some hue shifting to make it more interesting to look at.
The
human seems fairly well done but strays quite a bit from the original design. I'd like to mention the eyes again but if you were to want to keep the white eyes to monsters that could work but it wouldn't hurt to try it on the character too judging by the sketches. If I were to have to pick which human most closely matched the fish (which I think is the style you should be aiming for because it turned out really well) I'd pick the top right one. Here you used the "full outline" with some lightening towards the top. Colors seem pretty nice on this one, wouldn't worry about those.
I hope that cleared some stuff up for you. Those are all just my opinions but I do feel that there's a bit of work done before these will look like they belong together in a game. You're off to a good start though
Best of luck.
