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, then press A to advance to the next line of
dialogue and unlock the corresponding costume:

8-bit Girl: Press Up(2), Down(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(2),
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:* You must get the Reindeer Suit in Neverland
first.