Clif,
thanks for taking the time to read my post. I can't actually do more for suport than simply posting here, so I do the best I can.

Nardoni is pointing out something here, that I also forgot to mention, that the kickboxing thing maybe a possible means for her to "regulate" to some degree the behaviour of others?

Or is the kickboxing a means to get "invited" to some kind of "club" she wants to enter, or get a certain boy's attention? I don't know how old is she, does she have to take care of the younger siblings? Is it the only "physical exercise" thing she will do?
Too many questions, and I feel asking them here won't solve it. Try to sweeten her up with talking about things that interest her and then gradually turn the subject to this and see what will you find out.
good luck.