Hikari

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.