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.

Unlock Alabasta Stage:
Using Crocodile beat Story mode once.

Unlock Very Hard Mode:
In Event Battle clear Hard mode with more than 10 Characters.

Unlock Groggy Ring:
In Davy Back Fight mode win 3 events 3 times.

Unlock Marine HQ Stage:
Unlock ALL the characters completing their move lists in Training mode and then destroy ALL the pillars in the stage.

Complete the following tasks to unlock the corresponding characters

Unlock Don Krieg:
Clear Event Battle with Sanji.

Unlock Arlong:
Clear Event Battle with Nami.

Unlock Buggy:
Clear Event Battle with Zoro.

Unlock Kuro:
Clear Event Battle with Usopp.

Unlock Sir Crocodile:
Clear Event Battle with Robin.

Unlock Smoker:
Clear Event Battle with Luffy.

Unlock Bon Clay:
Clear Event Battle with Chopper.

Unlock Foxy:
Clear Event Battle twice with Chopper.