Girl, you know I looove this piece.
But this is a place for super srs critiquing, so here I go!

I think you have your concerns mixed up a little bit. Because of Hiccup's hair, he seems more 'cartoony' compared to Toothless, who has a nice range of colors. You shaded the ends of Hiccup's hair, but there is little to no shading from the scalp of his hair down. I know you're capable of shading that, because look at that beautifully rendered fur coat! I think if you go back in with the same shading method, it would help make Hiccup more realistic. You could even try to push the rim light of the hair to his right.

But that brings us back to your concern that Toothless is too cartoony-- My advice is to push the colors and their contrast. The wings have a really interesting dynamic in them, like light is trying to come through but just can't quite make it. I like that, but you can probably push it further :)

I think the best thing you can do to push the realism is to get nitty gritty with the details...zoom in on this picture and study the light-gray hexagonal pattern on his nose, ear-things, and legs. Your piece has very soft accents, but I think if you add in some sharp lines it could really help :D

One other tip, try adding some flecks of dust particles into the beam of light with a small, white brush. It'll really sell the illusion of light ;)

Hope this helps! <3