For me it was 2nd round Vexen (Sora's Story) and Lexaeus (Riku's story) the rest were relatively easy.
... With Kairi Having no experience with Defending herself & fighting the heartless, she would of thought she was a nuisance to Sora. So the smart thing to do was to stay behind on the islands. This turned out to everybody's favor such as Sora, Organization XII, Namine, etc. This was also useful for Kairi's promise to Sora that he would come back to the islands with Riku. OR the staff got lazy and decided to put Kairi on the islands because it would make the plot more simpler than it could of been. lol.
