How did the American Kingdom Hearts differ from the Japanese original?

The usual flow of 1990s and 2000s game localization ran one way: Japan first, America later, sometimes with cuts. The first Kingdom Hearts ran backwards. The game launched in Japan in March 2002, and when it crossed to North America that September, it arrived expanded. Three boss encounters in the English version are simply not in the Japanese original: the Sephiroth duel at the Olympus Coliseum, the Ice Titan match on the same stage, and Kurt Zisa, the great serpent of Agrabah's desert.

For Japanese players this was galling in a specific way: the most famous of the three, Sephiroth, was a Final Fantasy icon in a game sold partly on Final Fantasy star power, and Japan's own audience could not fight him. Square's answer closed the loop. Final Mix, four months after the American launch, re-imported the American content to Japan and then outbid it, with the Unknown boss, Proud mode, and the extended secret movie described on the Final Mix page.

The episode set the series' pattern for a decade: Japan originates, America expands, Japan re-releases the expansion with interest, and the rest of the world waits for a collection. When you find a Japanese Final Mix on a shelf, you are holding the receipt for that whole cycle: the one market that got everything, twice, and the only market that got it first.