Unlockables

Reverse/Rebirth Mode - To get Reverse/Rebirth Mode where you play through the story as Riku, simply beat the game once as Sora.

Reverse/Rebirth Mode Beat the game as Sora.

Special Cards - Unlockables

Ansem Beat Reverse/Rebirth mode and obtain from a chest in Twilight Town.
Axel Enemy Card Progress through the story
Demyx Enemy Card Atlantica: Key to Rewards after beating Reverse/Rebirth
Diamond Dust Beat Riku's story and obtain from a chest in Castle Oblivion.
Getting Mushu and Xigbar card Go to Hollow Bastion and use key of rewards and there will be two chests one containing Xigbar
Larxene Enemy Card Progress through the story
Lexaeus Beat Reverse/Rebirth mode and obtain from a chest in Castle Oblivion.
Luxord Enemy Card Agrabah: Key to Rewards after beating Reverse/Rebirth
Marluxia Enemy Card Progress through the story
One Winged Angel Beat Riku's story and obtain from a chest in Twilight Town.
Roxas card (Doubles attack strength) Go to Twilight Town in Sora's mode and enter the treasure room. There should be two chests with one of them containing the Roxas card.
Saix Enemy Card Traverse Town: Key to Rewards after beating Reverse/Rebirth
Ultima Beat Riku's story and obtain from a chest/shop in Castle Oblivion.
Vexen Enemy Card Progress through the story
Xaldin Enemy Card Monstro: Key to Rewards after beating Reverse/Rebirth
Xemnas Enemy Card Wonderland: Key to Rewards after beating Reverse/Rebirth
Zexion's Card Beat Riku's story and retrieve from a chest in Destiny Islands.

Theatre Mode - Beat the game once to unlock Theatre Mode, an extra option on the main menu wherein you may re-watch any cutscene in the game.