Defeat the following characters fifty times, then partner with them and go to Martha's place. Talk to her and select the second option. The day will end and your partner will change his or her costume. Note: Their hearts will also reset and they will use another deck type.
COSTUME - HOW TO UNLOCK Akiza's Academy Uniform - Defeat Akiza fifty times. Rua/Leo's Academy Uniform - Defeat Leo 50 times. Ruka/Luna's Academy Uniform - Defeat Luna 50 times. Yusei's Poncho - Defeat Yusei 50 times.