I'd imagine Yachiru is an incredible fighter as well. A large part of being in Kenpachi's 11th squad is having the ability to fight, and I mean with brute strength, not with demon magic or special Zanpaketou abilities. Remember way back when Yamichika was fighting Lieutenant Oba(sp?), and he said he would reveal his sword's abilities, but to keep it quiet since 11th squad hates flashy Zanpaketou techniques? So unless Kenpachi and the rest of 11th squad are lenient of having a weak lieutenant, then Yachiru is strong.

Plus what complich8 said .

I have a question though: how long will she continue to act like a little kid? I'd figure she's like the little girl from "Interview With a Vampire": child's body, adult's mind. Maybe she's perma-brain-damaged from all the violence she saw as a child.