One Piece - Grand Adventure - Platform: Gamecube

Another way to get Nico Robin (Mrs. Sunday):
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In 'Crocodile Adventure Mode', Nico Robin is the first ally you get. 
Get her, then win a battle with her.

Buggy:
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To unlock Buggy as a Captain in Adventure Mode, in Luffy's Adventure 
Mode, defeat Crocodile in Alubarna on the Alabasta Kingdom Sea Area, 
and recieve the rank "Top Pirate in Town", and before or after the 
credits, you will recieve Buggy as a Playable Captain in Adventure Mode.

Crocodile:
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To unlock Crocodile as a playable captain in Adventure Mode, in Luffy's 
Adventure Mode, defeat Crocodile in Alubarna on the Alabasta Kingdom Sea 
Area Map, and after you defeat him, get the rank "Top Pirate on the Island". 
Before or after the credits, you will recieve Crocodile as a Playable Captain 
in Adventure Mode.

Eneru:
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In Crocodile's adventure mode, get Eneru as your ally in the Jaya area, 
and win a battle using him.