I'm not the expert for this, but I'd say the shadow on the right side of his nose (his right) looks odd. Beyond that it mainly looks rushed. His lips, his nose, his throat, his chin -- they are okay, but I think it you put some more thought into them you could get them much better. Right now they feel flat. They could be catching the character of his face and the light much better.
from THE BOOK OF LIES -- a viking struggles to break the curse so he can join his family in Vahallas
In addition to thinking about what looks realistic, also think about what helps the art.
For example, in most cel shading, the neck is always shaded more or less completely. This is because it highlights the face and makes the image read clearer. In your image, there is a line of pink that runs straight from his forehead to his collar, which makes it hard to "read" his face.
Similarly, the direct sideways light you're using is normally used specifically to make a character seem sinister or two-faced. This is because the line of shadow divides the face in half. This is why most cel shading is done assuming a forward light source of some kind.
To soften the shading, a lot of cel shading uses an edge light on the dark side of the image, to make the image easier to read. See Persona 4 characters to see that in full force.
Basically, cel shading is not real shading. The way the shadow falls creates what is basically a line, and you have to consider whether that line helps or hurts your composition.
First off there is no reason to make your image that big. When I post stuff on my online galleries I generally don't go larger then 900 px tall. I'm not going to claim it should be the gold standard for online posting but 4960x7015 is overkill.
Here are some really good pictures of loki with similar lighting.
Your hair line comes down too far he needs more forehead. His nose flares out a bit at the bridge you indicated that on the right side you do not on the left side. It makes his nose look unsymmetrical. There should be more darkness under the eyebrow. You need to either get rid of the shading on the left side of the nose or add shading to the cheeks.
and also, the reason as to why he had more hair was because I tried to make his face look downwards, hence the ears being higher up than normal, but looking at it now... D:
And I'm not entirely sure about the shading to the cheeks, considering that I intended to do a single light source on his left to establish being only one light source. Wouldn't the shading on the cheeks be a bit confusing?
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Even though you only have one light source that casts a shadow over his right side, the light travels and then bounces back and highlights his right cheek and ear. Also there would be places on him that would cast darker shadows then other places. I drew up an overlay on my ipad to illustrate what I'm saying. Hope it helps
Its not exactly correct, there's some places around on the right side of his head around his ears that should have darker shadows, its very quick and dirty. Just remember that the shadows the light casts will reflect the shapes of his face, and those shapes are all 3 dimensional so they recede and pop out depending in the amount of shadow and light. Also remember that light travels and will reflects back, so remember to think about secondary lighting.
UltimateB:Even though you only have one light source that casts a shadow over his right side, the light travels and then bounces back and highlights his right cheek and ear. Also there would be places on him that would cast darker shadows then other places. I drew up an overlay on my ipad to illustrate what I'm saying. Hope it helps
This is exactly what edge lighting or rim lighting is.
Again, it's not precisely "correct", but that's okay.