First Impressions: Bold, compact, aggressive, cute~
=I think the design is simple but effective; seems very fitting for the character (I also like the subtle outfit variations in different situations). The spacing between light and dark elements should read well even in monochrome.
I think so, yes. His face in particular is consistently eye-catching~
If you do want to add interest to his design, I think a simple but recognizable element on the chest or head could work well (T-shirt logo, necklace, headband, hair streaks). I liked the scar in the first pic; you might try bringing that back...or giving him a new one.
First Impressions: Weird, unsettling, like a human Muppet
=He seems cartoonish by himself, but not compared to the other characters. And as for exaggerated...not really. =/ As fat characters go (I assume from the first pic...his body shape is a little inconsistent) he actually seems a little understated. That's probably what makes him uncanny...it's like you went for a goofy, funny-looking character but stopped short before you actually got there. So the resulting design kind of floats between normal and abnormal.
First Impressions: Rough, violent, fiery
=I decided to focus on the transformation. It's a little simplistic; it looks like all the original character gained was some markings on the skin and a lot of hair. The horns look really thin and flimsy in close-up shots; I feel like that could be improved upon.
I think you made that happen, but it could be improved with better musculature on the character. All the extra lines on the body (without muscle to back them up) just make the 'monster' form look emaciated, not stronger.
First Impressions: Strong, plain, like a gladiator
=The hair is kind of goofy...it reminds me of the hairstyles of Yu-Gi-Oh protags, but without the dynamic colors and shape language to back it up. =/ That aside, though; it's a solid 'tough guy' design, albeit a little on the generic side.
The 'floating' thigh pads stand out the most...in general, isolated pieces of armor (especially when they don't fully cover any one body part) tend to look odd. You could replace them with more subtle raised patches, or at least add more material...I recommend you look up some examples of leg armor to help with ideas.