For Single and Endless modes, while the game is loading, enter the following codes. You will be taken to the character select screen:
Up, Down, Triangle, Square, Start - Endless Mode
Down, Up, Square, Triangle, Start - Single Mode
Character: Carbuncle - Finish the HaraHara course with less than 4 or more than 12 fevers. You cannot continue. OR defeat Accord on the same course with your continues in multiples of 7. During the next stage you will fight Carbuncle. Defeat him to unlock him.
Character: Popoi - Finish the WakuWaku course.
All Characters and Cutscenes - At the cutscene viewer, hold the Square button and input Up, Down, Left, Right