One Piece: Grand Adventure Cheats

Unlocking Extra Costumes- Straw Hat Pirates
All Straw Hat characters but Vivi start with their 1P and 5P costumes, and in Robin's case the 4P as well. In general, 4P costumes will be found first, while 2P costumes appear later in the game and are more complicated to get. Any costumes unlocked in Adventure mode will be usable in every other mode.

In unlocking Luffy's 2P costume, make sure not to fight Crocodile until after you have travelled to Jaya and are prompted to finish Crocodile before proceeding, or your game will end at the Crocodile fight.

An easy way to beat Ace(Trace) with Chopper in the Graveyard of Ships is to use Chopper's "Health" attack (Jump+O) non-stop. Beware of rolling bombs.

Chopper 2P: Obtain the bonus item "Santa Hat" by beating Ace(Trace) with Chopper at a Wrecked Ship in the Graveyard of Ships.
Chopper 3P: Obtain the bonus item "Polo Shirt" by beating Chopper in 20 counts at Pepper Village in the Drum Kingdom Sea Area.
Chopper 4P: Obtain the bonus item "Panda T-shirt" by beating Nico Robin with Chopper at Erumalu in Alabasta Kingdom Sea area.
Luffy 2P: Obtain the bonus item "Pirate Coat" by beating Crocodile the first time at Alubarna in the Alabasta Kingdom Sea area with a super attack.
Luffy 3P: Obtain the bonus item "Baseball Uniform" by beating Luffy with more than 30% HP at Mustard Village in the Fusha Village Sea Area.
Luffy 4P: Obtain the bonus item "Cool Shades" by beating Buggy with an item at Orange Village in the Orange Village Sea area.
Nami 2P: Obtain the bonus item "Yellow T-shirt" by beating Buggy at Vermillion Town in Jaya Sea Area with Nami.
Nami 3P: Obtain the bonus item "Cheerleader" by beating Nami with more than 70% HP at Coco Village in the Roguetown Sea Area.
Nami 4P: Obtain the bonus item "Fashionable Sunglasses" by beating Chaser at Navy Base No.64 in the Inshore Alabasta Kingdom area.
Robin 2P: Obtain the bonus item "White Hat" by beating Ohm the first time at the Ruins in Skypiea.
Robin 3P: Obtain the bonus item "Kimono" by beating Nico Robinwith more than 50% HP at Misty Peak in the Drum Kingdom Sea area.
Sani 2P: Obtain the bonus item "Gaudy Shirt" by beating Don Krieg at Vinegar Village in the Jaya Sea area with Sanji.
Sanji 3P: Obtain the bonus item "Soccer Uniform" by beating Sanji without jumping at Carmine Town in the Orange Sea Area.
Sanji 4P: Obtain the bonus item "Necklace" by beating Chopper at Big Horn in the Drum Kingdom Sea area with Sanji.
Usopp 2P: Obtain the bonus item "White Bandana" by beating Kuro at Mock Town in the Jaya Sea area.
Usopp 3P: Obtain the bonus item "Chestpiece" by beating Usopp without getting knocked down.
Usopp 4P: Obtain the bonus item "Sunglasses" by beating Nami at Coco Village without knocking her out.
Zolo 2P: Obtain the bonus item "Goggles" by beating Arlong at Spice Village in the Jaya Sea area.
Zolo 3P: Obtain the bonus item "Judo Wear" by beating Zolo with a super attack at Red Town in the Fusha Village Sea Area.
Zolo 4P: Obtain the bonus item "Necklace" by staying alive for 30 counts against Hawk-eye Mihawk at Blue Town in the Syrup Village Sea area.

Glitch: Removal of Opponent's Supports
This is for battles where the CPU starts off with multiple support characters helping them. If you open the disc tray/cover as soon as "FIGHT!" appears on screen, you can battle as normal without their supports interfering. When the battle is finished, you need to close the disc tray/cover to let the game resume. NOTE: This does not cancel permanent effects, environmental changes, or supports that are summoned during the battle.