You can unlock more accessories for your characters by doing certain things.
You can unlock the 6th Prince, Kiefer Bergmann (Kiss Maxgaia in the JP version) after you beat the game once.
Go throughout the game without becoming anyones partner and at the end, Tony will dance with you at the ball. After you dance, and win, Tony will turn human and say he needs to return to his kingdom and asks you to accompany him because you are his one love. For the first ??? picture, turn Tony down to get the 'Princess' picture and for the second ??? picture, accept Tony's proposal to get the 'Tony' picture.