One Piece Unlimited Adventure - Unlockables

Bazooka Marine: Defeat the Bazooka Marine in the Seaside Zone.
Blue Demon General: Defeat the Blue Demon General in the Cave Zone.
Cluster Demon Soldier: Defeat the Cluster Demon Soldierin the Cave Zone.
Crimson Demon General: Defeat the Crimson Demon General in the Cave Zone.
Knuckle Pirate: Defeat the Knuckle Pirate in the Ruins Zone.
Marine Corps Chief: Defeat the Marine Corps Chief in the Jungle Zone.
Normal Marine: Defeat the Normal Marine in the Seaside Zone.
Pistol Pirate: Defeat the Pistol Pirate in the Jungle Zone.
Saber Pirate: Defeat the Saber Pirate in the Jungle Zone.
Sword Demon Soldier: Defeat the Sword Demon Soldier in the Cave Zone.
World Government Agent: Defeat the World Government Agent in the Ruins Zone.

Versus mode characters

Defeat the following opponents during Story mode to unlock that person in Versus mode.

Arlong: Defeat Arlong at the Cave area in Story mode.
Crocodile: Defeat Crocodile at the Cave area in Story mode. Note: He is the second Boss. To locate him, hit the wall with a character instead of a pick axe.
Eneru: Defeat Eneru at the Ruins Zone area in Story mode.
Mihawk: Defeat Mihawk at the Fossil Beach area in Story mode.
Smoker: Defeat Smoker just before you enter Fossil Beach in Story mode.
Spandam: defeat Spandam at the Ruins Zone area in Story mode. Note: He is the first Boss.

Transform into Overlimit Chopper in Story Mode
In order to do this, you must have completed 2 previous tasks:

Defeat Overlimit Chopper in Story Mode
Create the Rumble Ball

After achieving those requirements, you will now be eligible to transform into Overlimit Chopper. To do so, you first must be in a boss fight. While in the fight you have to use TWO Rumble Balls wait for one to run out, heal SP gauge, take another one. While you are using the second Rumble Ball, the enemy has to kill Chopper. Instead of dying, like he normally would, a cinema scene occurs. The scene shows Chopper taking a third Rumble Ball and he transforms into his Overlimit form. He gains a new HP and SP bar. He isn't able to use items.