Thick outlines for the mouth help the expression pop against the rest of the face.
Usually, the eyes are narrowed, closed, or "half-lidded," suggesting a sense of dominance, mischief, or overwhelming pleasure. hentaismile
For digital illustrators, mastering the Hentaismile is a lesson in It requires breaking the traditional rules of facial proportion to achieve a look that is "unnaturally" expressive. Thick outlines for the mouth help the expression