I wanted to post some Kim art from fnaf, but there's no tag for her-

If there isn't any tag for the character, just add it yourself to the image. That way the tag will be created. You can create any tag you want that way.

Then, go to the web's Tags section (rule34.xxx/index.php?page=tags&s=list), and search the tag you just created. You'll be able to edit it, and change it from "general" tag to "character" tag.