The artist coffeestain's tag is shown as being a character instead of an artist on all of their images? Pretty sure I can't fix this.

Fixed. Use the prefixes artist:, character: copyright:, species:, or general: if a tag is in the wrong category.

As long as there are less than 100 posts with it, you can change a tag's type just fine. Chances are somebody named a character coffeestain and didn't realize that character: isn't a part of the tag here, contrary to other boorus like derpibooru, where it is.