Balboa is a friend of Joukyuu Kunitoshi who uses a Wazato (a slanted sword), which she wields proficiently in battle. For reasons unknown, she is not normally allowed to enter Japan. Later it is stated she has a bounty on her head for stealing a kabuki script.
She and Joukyuu seem to have know each other before the story began as when they meet he commented on how much her body has grown. She also might have been train by Joukyuu or at the very least he gave her advice about martial arts (she says during one of her fights that Joukyuu once told her that martial arts were to be done with both arms and that she can wield her sword with her left as well as her right).