She has skills that far exceed what you'd expect from her appearance. Maybe it's because she is a bit strange, mature for her age, or doesn't look like her age at all. Vaguely similar to a Tyke-Bomb save she isn't necessarily groomed by a Big Bad, and may well be a Big Bad in her own right or at least The Dragon. Might be the fighting powerhouse on the good guys side and can be mistaken for just the Tagalong Kid until she unleashes the Waif-Fu.
The character may be quite cute and is often smart enough to use that to her advantage, quickly manipulating anyone stupid enough to fall for her act. Her actual personality will usually be very clever and cynical, though. She may also be more innocent or good-natured, but no less charming so that she generally gets her way. Often extends to what she's wearing, if she cares at all.
If put into serious danger she can often show an absurd amount of power and will easily deal with any annoying perverts in the most casually devastating way possible. Often a practitioner of Waif-Fu, and can be an example of Small Girl, Big Gun or Girl with Psycho Weapon. Can be a Cute Bruiser and/or a Pint-Sized Powerhouse, but doesn't rely on brute strength. If she doesn't have ridiculous strength, she would be Weak, but Skilled. Most noticeable in characters who look very young, since many characters tend to catch on to overpowered teenagers much quicker.
Compare Enfant Terrible. May belong to a Badass Family. May grow up to be a Lady of War, although often with a touch of Femme Fatale. If lacking in charisma, she would be an Action Girl, or possibly Dark Action Girl.
Contrast Badass Adorable and Tyke Bomb; the distinction being that the Little Miss Badass is never restricted by a guileless disposition and can occasionally be found reveling in her own badassery, veiling her ruthlessness only when it suits her.
See also Troubling Unchildlike Behavior, which some of these characters engage in.
