The Japanese version contains mostly-finished data for the English localization of the game, which was still unfinished by the time it was released in Japan. By using a hack to trick the game into running in English, most text files and voice lines will be in English, and the controls will change to reflect the NA/PAL standard. However, the cutscenes will still be the uncensored Japanese animations with their respective original lip-syncing, and Roxas will use older voice clips from the original Kingdom Hearts II instead of the final English release which uses voice clips from Ventus from Kingdom Hearts: Birth by Sleep instead.
Contributed by DefinitiveDubs
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In the original Kingdom Hearts II on PlayStation 2, Kairi's underwear can be seen by maneuvering the game's camera angle. This was removed in the HD version.
Contributed by ProtoSnake