A character with a mouth that is drawn wide--often shaped like a stretched-out "W" or a } rotated 90 degrees to the right--but which is usually only ever partially or slightly opened. This creates the visual impression of a snakelike mouth.

Also qualifying for this are especially wide [[:>]]-style closed mouths on a character who is already known for this sort of thing.

Notable examples of this are [[Ichimaru Gin]], [[Orochimaru (naruto)|]], and occasionally [[Koko Hekmatyar]] (all of whom, unsurprisingly, have some connection to snakes)
snake mouth