Enter the Single or Endless mode starting menu, then quickly press Up, Down, Triangle, Square, Start to unlock all characters. If you entered the code correctly, you will hear a sound.
All Characters And Intermission Sequences
At the options menu, choose the "Gallery" selection. Highlight the "View Cutscenes" option, then hold Square and press Up, Down, Left, Right to unlock all characters and intermission sequences.
Play As Carbuncle
Play in HaraHara mode. Either get less than four or more than twelve fevers without any continues. Alternately, complete the game with your number of continues as a multiple of 7.
Play As Popoi
Successfully complete the WakuWaku course under the normal difficulty setting.