Corrected entry: Ansem cannot open the last keyhole because Kairi's heart is not with her, it's with the people she loves. But by that logic, Belle's heart would be with the Beast, Snow White's with her prince, Aurora with Philip's, and so on. Yet the only princess who isn't responding to Ansem is Kairi.
xSpoonxManx
24th Jan 2008
Correction: Kairi's heart was literally not with her. In the scene on Destiny Islands when Kairi stands in front of the door and is blown towards Sora, her heart actually enters Sora. That is why when you see Kairi on Captain Hook's ship she is virtually lifeless.
xSpoonxManx