What makes Kairi one of the 7 princesses in Kingdom Hearts what is she princess of

Kairi is one of the seven princesses in kingdom hearts because they explained in the first game that the seven princesses are seven purest of heart that are never tainted or twisted by evil. Kairi has a pure heart and that's why she is one of them. and she has ties to the keyholes without the either of the seven princesses the keyholes will not be complete until the seven princesses are complete.