Concept art tic: it always better to develop the face of the character away from the form and body shape. Of course there is a way to render high quality sketch from scratch, but you will probably be told to develop several more. That is why it is better to draw the faces separately. A lot of the personality is put in the faces and their emotions.

Here are the faces.

character concept design

I'm beginning with outlines this time.

character concept design



character concept design



character concept design

N2. I really like how the 2nd done came out: started with blocking the values from start.

character concept design



character concept design

N3. The 3rd one came out really good too. It has plenty
  
of details on his mask.

character concept design



character concept design

N4 - Number 4 has a little bit long neck, but in the rest he turned out well. The same technique was used on N4: blocking the colors from start and just playing with the value.

character concept design



character concept design



character concept design

...A little skin reflection. Better to use some texture on the brush - like I didn't

character concept design



character concept design

And here we are: we've just created 4 character concepts. It was a lot of fun to do this. I hope you've enjoyed it like I did and really learned something. So practice a lot to master your skill in painting; the speed will come in time.