Unlock Sixth Class
At the Class Selection screen, highlight the classes and press the R Trigger in this order - Pirate, R Trigger, Thief, R Trigger, Warrior, Trigger R, Archer, R Trigger, Pirate, R Trigger, Warlock, R Trigger, Thief, R Trigger, Warlock, R Trigger, and Thief, R Trigger.
Note: This must be done quickly in order for it to work.