To unlock Ussop as a Playable Captain in adventure Mode, defeat the final boss in Luffy's, Buggy's, Crocodile's, AND Chaser's Adventure Modes. After the credits of the last Adventure Mode(No order is required), you will unlock Ussop as a Playable Captain. In Ussop's Adenture Mode, you can use make a team of ANY Pirates you have unlocked!