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.

Politan Suit : Press R3(3), Right, Left, Y, X, LT, RT, LB. Note: Earn the Reindeer Suit in Neverland first.
Gemaga shirt: Press RT, LT, LB, RB, Y(3), X(2), Up.
Platform Logo shirt: Press RT, RB, R3, L3, LB, LT, Right, Left, X, Y.
8-bit Girl shirt: Press Up(2), Down(2), Left, Right, Left, Right, Y, X.
Hirakou shirt: Press X, Y, LB(2), RB(2), L3(2), Up, Down.
Club Famitsu shirt: Press Y(2), Up(2), X(2), Left(2), LB, RB.