Successfully complete the game, then start a New Game+ under the Hard difficulty. Open the treasure chest on the Sunny with the key to lockers. Go to girl's room to change the costumes of your characters.
Hard difficulty
Defeat the final Boss on Island 5.
New Game+ mode
Defeat the final Boss on Island 5. This will also create a saved game file for One Piece Unlimited Cruise: Episode 2.
Special combined attack
Complete all 50 rounds in Survival mode. A chest with a certificate will appear. Nami, Usopp, Robin and Brook can now perform a special combined attack.
Survival mode
Read the monument on the last island.
Unlockable Characters/Modes
Beat certain bosses on the right difficulty to unlock them as playable characters in versus mode.
Alternate costumes...