I will claim the best secret character (since Baiken), one so powerful her very fists rend Heaven and Earth. From Senran Kagura, I claim
Daidouji!
She's actually blonde, but when she transforms into her shinobi mode she looks like a classic boncho. Her visual design is in part inspired from JoJo, obviously, and
She's also a serious minded, goodhearted warrior with delicious abs AND she's a 24yr old high school girl.
Yeah, she refuses to graduate ninja academy until she's the strongest or something, so she's technically a high school girl.