Beat the game to unlock New Game+ mode. Additionally, in Chapter 2, you can enter ReanBell's (Leanne's) bedroom and press [Action] at her wardrobe to activate a secret button. It will allow Reanbell to remain in her current costume instead of changing into the pig T-shirt in the cinematic.
During or after Chapter 7, select Leanne and look at the wardrobe in her room. Enter one of the following codes when she says "Hmm, I think that outfit is in here...", then press X to advance to the next line of dialogue and unlock the corresponding costume:
HOW TO UNLOCK
8-bit Girl shirt
Press UP(2), DOWN(2), LEFT, RIGHT, LEFT, RIGHT, Y, X.
Club Famitsu shirt
Press Y(2), UP(2), X(2), LEFT(2), LB, RB.
Press RT, LT, LB, RB, Y(3), X(2), UP.
Press X, Y, LB(2), RB(2), L3(2), UP, DOWN.
Platform Logo shirt
Press RT, RB, R3, L3, LB, LT, RIGHT, LEFT, X, Y.
Press R3(3), RIGHT, LEFT, Y, X, LT, RT, LB. Note- Earn the Reindeer Suit in Neverland first.