I think the shading is too flat and not very consistent. For example, the running guy's hair looks like it's being lit by an overhead light, while his spear looks like it's being lit from a light coming from the ground.

I would try to use shading to try to make their forms more apparent. Right now it kind of looks like they're made of paper, ( except the Night King's head, it does already have some form to it).

Also, every color should have shadows/midtones/highlights. The running guy's hair doesn't have any highlights, while his face is almost completely lit up (there should at least be shadows under his hair). Both his eyes also seem to be visible which is kind of weird from that perspective, since his nose should be in the way of his other eye.

And if you want to add extra detail to the armor I would use the highlights to add a sort of texture to the armor, instead of just straight lines.