Final Keyhole

The Final Keyhole is the last seen keyhole in the first Kingdom Hearts. It is in Hollow Bastion where the seven princesses are in their dormant states and their hearts are opening Kingdom Hearts presumably. It doesn't truly unlock until Kairi's heart is restored by Sora after the second fight with Riku. Sora had given up his own heart to save Kairi's and the other princesses. It was revealed by Maleficent when she spoke "The princesses of heart are all here. It must be her." that Kairi was also another key to opening the heart-shaped keyhole.