This is mostly concerning the first picture.
Since you seem to focus most on 2D character/cartoony designs, I think the strength of your line needs to improve. There is such a thing called "line weight" which more or less refers to the emphasis put on a single line. In other simpler words: Making some lines darker and wider than others. Most of your sketches have the same line weight, which can make the picture very simplistic in impact, because the veiwer's eye is not being drawn to anything in particular. This could also be due to your very light sketchy/messy style of drawing, but there is such thing as a "clean" sketch. :]
I see attempts of shadow and shading. I'm sure you were not focusing on shading when you sketched, but I shall give some tips that I think you could use anyway. Try to use as much as the actual pencil as you can, when you shade. I know blending with your finger can sometimes be the easiest way, but don't use your fingers to do more than ... oh I'd say 30 percent of the smearing. Esspecially in just regular pencil (obviously things like charcoal and such are different).
Reason why I say that, is because I think a character sketch looks alot more ... whole and the same when the shadowing texture matches the line texture of the actual figure. Get what I'm saying? Maybe you don't...haha. Oh well.
Now for the actual picture. You really don't have any serious anatomy problems. But I'm just going to point out some things.
The skull seemed slightly too small
to me. I have a habit of doing big heads, though. That or maybe the ROOT of his hairs could of been placed higher upon his head. I'm not saying make his hair shorter, but raise where the start of his hair spikes started on the top parts of his head.
The shading around the eyes and nose is nice, and it looks fleshy. Though I couldn't quite put my finger on it, something about his jawline ... it seems too narrow? I mean, I know you were probably trying to create his unique appearance when you drew his facial features, but his jaw condyle (the joint of your jaw which connects to your skull) seemed to high and rigid.
The muscles in his throat should be slightly curved down to where they meet up with the clavicle. They are actually the SCM muscle (sternoclediomastoid in long terms, yay anatomy!) and the SCM sort of wraps around your neck a bit.
Now, he has way too many teeth to be correct, but I like it.