the princesses dont fight they r just there making up the Realm of Light while the 7 lights (keyblade wielders/masters) fight the battle against the 13 darkness' to protect the princesses because if the princesses are taken by the 13 darkness' then there will be no Realm of Light but i understand what you mean but it does make sense what your saying Nomura probably did set it up like that so that Kairi would equal it out her being a Keyblade Wielder/Princess which brings me to a question of if she is to fight and if she falls in battle will the Realm of Light cease to exist? or will it become a little darker?