There are also Secret Portals that open up later in the game - these allow you to refight bosses, albeit much harder than the original fight. These portals are always available once you are far enough in the game, and are located in the boss rooms that you fought them in originally. I...
Like Nomura tells Shimomura: “I want a track called The Other Promise which is the VS Roxas theme and Roxas’ story is like this...” back to topic: I’m really disappointed with the name of the 13 Seekers theme boss theme. It’s literally “Replicas”. I mean, yeah ...