My artist has been drawing, then digitally re-creating, a few creatures for a secret project I'm working on. He's also a RL friend, and wanted some feedback.
He should study lighting some more, and work on how to consistently describe form with shade and contour. In both cases the form that the lighting implies isn't consistent with that of which the contours imply; making the forms look very two-dimensional. There looks to be a tendency to stick most of the shadows close to the outline when it would be more sensible for the shadows to occupy much more of the shape's surface area.
Your friend should also try to be more aware of the fact that different materials reflect light in different ways. Slime has a comparatively smooth surface and thus the light that it reflects is much more concentrated, as opposed to a more matte material which scatters the light it reflects a lot more. This means that the blob should have much brighter, sharper highlights in comparison to the second drawing, which I can only assume (due to the lighting) is covered by a dull surface.
Do remember that slime is translucent, and allows some light to pass through it, diffused. Since slime is rather shapely and not square or box-like it would make sense for the outer edges of the form to be considerably lighter than the inside, as there is less mass there in the way of incoming light.
Not awfully important but rarely in life do you ever have light come from a single direction - light that has been reflected can find its way onto an alternate side of an object. Or perhaps another secondary, less intense light source may act upon the scene. While not necessary and rather situational it can help apply some more depth to your object.

Some work on a theoretical maskless Pyro, based on some of the
official concept art. I ended up making him look much more stone-faced but I think the official concept is more faithful to the shape of the head in-game. Might take some shots of the actual model and try again based off that.
Anyone have any clue of the Pyro's possible ethnicity based on the concept art?