You might want to try a hard brush, because this seems unintentionally blurry. I believe its from using a soft brush in photoshop/painter. On the edges of the characters try using the hard brush to give the characters more defining edges.
It looks like you have it working on all the black colors, but none of the lighter colors, which makes the piece seem inconsistent. But overall the shading is done well and is nice and smooth. Just work on the edges.
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