During or after Chapter 7, select Leanne and look at the wardrobe in her room. When she says 'Hmm, I think that outfit is in here...' enter one of the following codes and press A to advance to the next line of dialogue and unlock the corresponding costume.
Unlock Politan Suit:
Click Right Analog-stick(3), press Right, Left, Y, X, LT, RT, LB (earn the Reindeer Suit in Neverland first).
Unlock Platform Logo:
Press Left, Up, Right, Down, RB(2), LB(2), click Left Analog-stick.
Press X, Y, LB(2), RB(2), click Left Analog-stick, press Up, Down.
Unlock 8-Bit Girl:
Press Down(2), Up(2), Left, Right, Left, Right, Y, X.
Unlock Club Famitsu:
Press Y(2), Up(2), X(2), Left(2), LB, RB.
Press RT, LT, LB, RB, Y(3), X(2), Up.