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 A to advance to the next line of dialogue and unlock the corresponding costume.

Costume	How to unlock
8-bit Girl	Press Down(2), Up(2), Left, Right, Left, Right, Y, X.
Club Famitsu	Press Y(2), Up(2), X(2), Left(2), LB, RB.
Gemaga	Press RT, LT, LB, RB, Y(3), X(2), Up.
Hirakou	Press X, Y, LB(2), RB(2), click Left Analog-stick, press Up, Down.
Platform Logo	Press Left, Up, Right, Down, RB(2), LB(2), click Left Analog-stick.
Politan Suit	Click Right Analog-stick(3), press Right, Left, Y, X, LT, RT, LB. Note: Earn the Reindeer Suit in Neverland first.