mouths are often depicted as small, usually rendered with one line on the face. a fang peeking from the corner of the mouth indicates mischief or feistiness (unless, of course, the character has fangs normally). a cat mouth (like a number "3" rotated 90° clockwise) replacing the character's normal mouth, and usually accompanied by larger eyes may also represent mischief or feistiness (a notable exception being konata izumi from "lucky star", whose usual mouth shape is this).