Had this so many times.
Your choices are - complement the art, commission that artist so you can have a piece of art that has that character included, or, and this is the one lost on some people - shut the fuck up.
James Marsh
Just FYI, artists tend to find it very annoying when your only response to their artwork featuring lots of characters from a series is "where's x?"
At least complimentary the piece or something? Or have a think first why a character maybe didn't make the cut? It just feels dismissive.