In the game you can unlock the following characters:
Clear Event Battle with Zoro: Buggy
Clear Event Battle with Usopp: Kuro
Clear Event Battle with Sanji: Don Krieg
Clear Event Battle with Nami: Arlong
Clear Event Battle with Luffy: Smoker
Clear Event Battle with Chopper: Bon Clay
Clear Event Battle with Robin: Sir Crocodile
Clear Event Battle twice with Chopper: Foxy