Hakuno is a flower maiden who's kept as a pet to high-class noblewomen(Nero and Tamamo, duh). BB is also a flower maiden, but presumably with some weird magic/not-quite-real shit going on. Hakuno helped her through her heat, which led to BB going ...BB for her. I really don't know how exactly BB's general everything would work in a flower maiden AU, and there would probably have to be some magic, but at the very least she kidnaps her so she won't have to share her and can be hers forever.

Obviously Nero and Tamamo mount a rescue mission, and Hakuno is plucky, and Altera is probably there too. Maybe she's the stoic barbarian who kidnaps innocent courtesans to be her brides? IDK.