Red-haired Shanks
Beat Story Mode with Luffy on the very hard setting.
Characters
Clear Event Battle with Zoro - Buggy
Clear Event Battle with Usopp - Kuro
Clear Event Battle with Sanji - Don Krieg
Clear Event Battle with Nami - Arlong
Clear Event Battle with Luffy - Smoker
Clear Event Battle with Chopper - Bon Clay
Clear Event Battle with Robin - Sir Crocodile
Clear Event Battle twice with Chopper - Foxy
Unlock Very Hard Mode
Clear Hard mode in Event Battle with more than 10 Characters.
Davy Back Fight Minigame Unlockables
DBF = Davy back fight
DBF 4 times won Groggy Ring
DBF 10 times won Donut Race
DBF 18 times won Crash Rush Boxer (the box breaking)
DBF 26 times won Snow (getting rid of snow)
DBF 29 times won Breaking Krieg's Armor
DBF 34 times won The "Fix" sign
33 DBFs won + All support char's voices unlocked; and play DBF once more to Unlock Sanji & Zoro voice (announcing)
Unlock new costumes for The Straw Hats
After unlocking Shanks and Mihawk, complete the entire move set for all The Straw Hat Pirates in Training Mode to unlock a third costume for them inspired by the Color Walk books.
New Title Screen
Unlock all the characters to reveal a new title screen that features everyone.
Stages
Maxim Stage - Clear Event Battle with Enel.
Groggy Ring - Win 3 events in Davy Back Fight mode 3 times.
Marine HQ stage - After unlocking EVERYONE complete their move lists in training mode and after doing so destroy all the pillars in the stage. Do this with every character to unlock the stage.
Extra Characters
Smoke - Successfully complete the game in event mode as Lufi, Nami, Zolo, Sanji, and Usop. Then use Lufi to complete event mode again.
Tashigi - Successfully complete the game in event mode as Lufi, Nami, Zolo, Sanji, and Usop. Then complete event mode again with each of those characters except Lufi
Aokiji - Clear Event Battle with Luffy, Zoro, Nami, Usopp, Sanji, Chopper, Robin, Buggy, Kuro, Krieg, Arlong, Smoker, Bon Clay, Crocodile, and Foxy. Then play Event Battle again with Luffy and beat Aokiji with a level 2 special to unlock Aokiji.
Enel - After clearing event battle with Luffy, Zoro, Nami, Usopp, Sanji, Chopper, and Robin complete some of there move lists in training mode. A picture should flash by of Enel challanging you, go to Grand Tours (tournament mode) and just play a regular tournament, one of the characters will be replaced with Enel, beat Enel and win the tournament to unlock Enel.
Shanks - Clear Event Battle with Luffy on the Very Hard difficulty after unlocking all previous characters including Enel and Aokiji.
Mihawk - Clear Event Battle with Zoro on Hardest difficulty. Beat Mihawk with Zoro's level 2 special after getting hit with Mihawk's level 2 special. You must unlock all other character before this including Enel, Aokiji, and Shanks.
Ms. Sunday (Nico Robin) - Beat normal with all characters, then beat it again with Kuro, or, beat normal with Kuro last.
Tony Tony Chopper - Beat hard with Nami, or beat hard mode with every character, then Nami.
Miss Wednesday - Press Square, Square, Triangle, X, X, Circle while the credits are rolling at the end of the game. A sound will confirm correct code entry.
Pandaman - Press Circle, Circle, X, Triangle, Triangle, Square while the credits are rolling at the end of the game. A sound will confirm correct code entry.
Mr. 2 Bon Clay - To unlock Mr. 2 bon clay, the following conditions must be met. Characters one thru fourteen must be unlocked - that is, the default characters plus four hidden characters, including Ms. Sunday, Tony tony chopper, Red Haired Shanks and Hawk - Eye Mihawk. Then, use Sanji in Story Mode (at any difficulty level) and enter this secret command while the ending credits are running:
Hold L1 and press X-X-O-X-LEFT-X-O-SQUARE-X-O-TRIANGLE-O-X-R1-R1-LEFT
note: the Mr. 2 Bon Clay code has been reported several different ways- there may be different codes for different versions of the game, but this is the one that has been submitted most frequently.