those other sprites are very very nice. everything has just as much character as you'd expect from the boss char.
Main guy looks a bit too much like your average Gunstar clone for my taste though, considering how captivating the enemies are....I'm afraid this could be a game where the enemies are far more charming than the protagonist.
hm....I have more issues with the big guy's contrast now that I've seen the rest, quite honestly as charming as his design is he looks like from a lower quality game than the rest.
He looks a little off-key between the little guys, his whole shading has a more gradient feel to it because you applied so many shades on him, I imagine you did it because he's big and you knew you needed more "detail" and make his shading smoother.
Dont think that a lot of shades applied like a gradient all over help create detail....they are a tell sign of ignorance on lighting. Obviously in your case it must be lack of experience with big sprites, tiny ones are superb.
Almost feels like you're aproaching the big guy as a bunch of small sprites mashed together, why dont you try sizing hs outline down, quickly reshading him completly, then resize back to this scale and compare what you get? I'm pretty sure you could get the basic way light goes right if you did it in tiny first.
Another reason I think this isnt working out, is you're not sure of how to do the metal texture.
Check
-Megaman X4 sprites (bosses mostly) for refference on how to do his sci-fi robot parts
http://www.sprites-inc.co.uk/files/X/-Metal Slug for the tank-like parts
http://www.mslugdb.com/main/gallery/sprites/ Shoulderpads
-why are the higlihts on the top? put them at the botton to emphasize the / angle of the plate they're on.
Balls.
-Ok, balls.Balls with stuff on them, and balls cut in half are all shaded the same way as any other ball.You have a lot of ball parts that you shade incorrectly because they have stuff on them or they are cut in half. Balls disperse light equally from the spot in which lights hit them; if you looked at a ball from the lightsource it should look pillow shaded, but as we're not doing that here it should be from an angle. Look at MegamanX4 sprites, they have lots of examples of how to do this.
-You on the other hand change the way light disperses if there's something on top of the ball...if it's only the botton of the ball, all sorts of things are distracting you.
Neck/tail
-Up that contrast, make the lower parts darkened (like the starship! it has a similar cone/cyliner shape)
-dont do small highlights on fur, that makes them look like speculars! if you have highlights make them abundant and dont contrast them with the midtone.
-why are there shadows right below the column? the fur is fully being hit by light there, and there arent any casted shadows!
Bone Plates
-still looking like Petals! those dark cracks look like veins for petals! plates wouldnt have cracks trough the middle. make them a little round so they cast shadows, and if you NEED to put some detail give them dents, like a stegosaurus should have!.
http://en.wikipedia.org/wiki/Image:Ba%C5%82t%C3%B3w_Park_Jurajski_001.jpgArms
-they pretty much just lack contrast! dont get shy because you're doing big sprites! also, move the speculars just a little to the left and up.
Brain.
-The lowered water level isnt working, it is a distraction! you must HAVE the sensation that it is in the water before you can have a underwater/out of water contrast!
-do it with shading, make the color of the liquid match the color of the brain very closely, maybe even use the same pallete. DARE to take away detail, give a LOT more room to the middle tone, use all the shades to very clearly show the volume of a brain, then IMPLY some brain texture (noodle like thingyies :p). that helps give the unerwater effect by making the liquid look thick.
- if posible, make some speculars (NOT WHITE BLOTCHES, better do nothing than doing that hahaha) or scratch effects to emphasize the glass containing it.
- if you want bubbles you'd have to make them far more abundant, but since they are always moving I think they'd be a hassle.
Legs
-the balance concerns me, the right leg looks like it's too far at the back to support him, as if he would fall down...but I ALMOST feel like this is justfied by style....think about it.
-dont highlight his right kneepad, it is in full shadow. put some highlights on the lit area of the left tigh...
Personal preference.
-I dont care for outlines, they are a big headache when animating specially if they have AntiAlias/Selout and I dont think they're worth the trouble at all, I would avoid them.
Finally, Linkage:
http://www.itchstudios.com/psg/art_tut.htm I have a feeling this might be of great use to you.