quote SargentGamenut

also I think there is a arena where you can fight with and against any character well that's what I made out from YouTube as a guy was fighting 5 sephiroth's and Terra's with cloud

He hacked the game. Tons of people on YouTube do that.
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Good luck playing it.It is import stupid.
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quote Garkaz

Source

+Recoloured Heartless.
+Extra difficulty mode, Critical Mode (HP and MP gain sliced in half, and you gain 50AP and several skills at the beginning of the game, including EXP ZERO, which technically lets you reach Xemnas at level 1)
+You can now beat up Roxas and the rest of Org XIII who died earlier.
+New form, LIMIT FORM.
+New skills, Round Break, etc.
+Super tough extra boss. Yes. He's WAAAAY tougher than Sephiroth.
+New sidequests.

These are some I can think off the top of my head.

Oh yes, plus Final Mix comes with a 3D, ps2 version of Chain of Memories.
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