Instructions
Turn on the PSP system and launch "Dissidia Final Fantasy" from the Game section of the PSP's XrossMediaBar.
Highlight the "New Game" or "Continue" option from the game's main menu and press the "X" button. If you select the "New Game" option, follow the onscreen instructions to create your game profile and complete the game's tutorial to navigate to the game's profile menu.
Highlight the "Story Mode," "Arcade Mode" or "Quick Battle Mode" from the game's profile menu and press the "X" button. If you select story mode, you will only be able to play as the Onion Knight during the Onion Knight's story sequences. If you select arcade mode, you must wait until you reach a battle that features the Onion Knight to use him. If you select quick battle mode, however, you can select the Onion Knight from the character select screen.
Highlight the Onion Knight's character icon from the character select screen if you selected to play quick battle mode. Press the "X" button to select the Onion Knight for your battle. If you selected either of the other two battle modes, you have no control over which character you use at any given time.