Bizkit047, one of the greatest KH players, has apparently found the code that lets you play as Riku.

YouTube - KH2FM - Riku Vs. Lexaeus

Some interesting things about Riku:
-He can't do ground combos, only air combos
-He's invincible for some reason
-He can't do finishers
-You have to use the Shortcut menu when fighting as him, otherwise the game will freeze
-He seems to be unable to use any ability other than Attack, such Dark Aura