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.
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.
Successfully complete the WakuWaku course on the normal difficulty setting to unlock Popoi.
Either get less than four or more than twelve fevers without any continues in HaraHara mode to unlock Carbuncle. Alternately, successfully complete the game with your number of continues as a multiple of 7.