One Piece: Gigant Battle Cheats

Unlock modes
Complete the Thousand Arena (the 2nd option in the main menu) in the following difficulties to unlock new options.

Battle Royale Very Hard: Complete Battle Royale in hard difficulty with 5 characters.
Taiman Battle Very Hard: Complete Taiman Battle in hard difficulty with 5 characters.
Vs 100 Battle: Complete Battle Royale and Taiman Battle in very hard difficulty with 5 characters.

Last Island
Nightmare Island: To unlock Nightmare Island you must clear every level (not mission) at least ones

Unlockable Supports
Here's a list of some supports and how to unlock them

Akainu: Complete training and 4 man battle royale with him
Aokiji: Complete training and 4 man battle royale with him
Brook: Sabaody/The start of an adventure/Clear with Luffy (Brook)
Chopper: Complete training and 4 man battle royale with him
Cobby & Herumeppo: Complete Training with 12 characters
Eiichiro Oda ( guy with the fish head): Land Festival/What if the eagle lands on the ring into the sea/Pass with Jinbei (Camie,Arlong & Hatchan
Franky: Sabaody/A little to g blind!/Clear mission with Luffy
Garp: Last Resort/Nogadobatoru/Clear with Luffy
Kizaru: Complete training and 4 man battle royale with him
Luffy: Complete training and 4 man battle royale with him