At the Start Screen hold R and input these codes for the 4th costumes. If the code has been entered correctly you will hear a confirmation noise.
Unlock Luffy's 4th Costume:
Press Up, X, Up, X, B, A, A, Up.
Unlock Sanji's 4th Costume:
Press Up, X, Down, X, Y, B, Y, X.
Unlock Usopp's 4th Costume:
Press Right, X, Right, X, A, Y, B, X.
UNlock Zolo's 4th Costume:
Press Down, X, Down, X, B, A, A, Y.
Unlock Nami's 4th Costume:
Press Left, X, Left, X, Y, Y, B, Down.
Unlock Chopper's 4th Costume:
Press Left, A, Right, X, Y, Left, Right, B.
To be able to enter the following codes you have to clear Story mode using any character on an any difficulty setting. After the last opponent has been defeated and while the credits are scrolling up the screen enter the code to unlock the corresponding character. Only one code can be entered after each time you clear Story mode.
Unlock Arlong:
Hold L and press Down, Down, X, X, A, A, Down, Down, Down, Down, Down, Left, Down.
Unlock Buggy the Clown:
Hold L and Press A, Right, Right, Right, Right, Right, Right, A, A, A, X, X, X.
Unlock Chaser:
Hold L and press B, B, X, X, A, A, Down, Down, Up, Up, B, B, B.
Unlock Chopper:
Hold R and press B, Down, L, Up, A, Right, X, Down, X, Down, Y, Right, A, Left, L, X.
Unlock Don Krieg:
Hold L and press Down, Down, B, B, B, B, B, Right, Right, A, A, A, A.
Unlock Kuro:
Hold L and press X, X, X, X, Left, Left, Right, Right, Right, Y, Y, Y, Y.
Unlock Mihawk:
Hold R and press Right, L, Up, B, Up, L, Right, Y, Left, X, Left, A, Down, A, Left, L.
Unlock Ms.Sunday:
Hold R and Press Up, Y, Up, A, Down, X, Up, B, Right, B, Left, A, Right, Y, Down, L.
Unlock Shanks:
Hold R and press Right, B, Left, A, Left, L, Right, B, Right, L, Down, A, Left, B, Right, Y.