After unlocking all the other characters above win the game with Wiper and Monkey.D.Luffy on Hard difficulty setting. (Refer to unlocking Wiper)
Hina
Once you've gained Enel, win the game with Usopp on Hard difficulty setting.
Luffy's Crew
For those who are wondering, Luffy's crew consists of Monkey.D.Luffy , Roronoa Zoro , Nami , Tony Tony Chopper , Usopp , Sanji and Nico. Robin.
Luffy's group's fourth costume
Win Grand Battle (three rounds, hard difficulty, unlimited time) with Luffy's group's third costume against any CPU player. Note: You must win all three rounds with a level 2 special.
Luffy's group's third costume
Successfully complete training mode as Luffy, Zorro, Nami, Usopp, Sanji, Chopper and Robin.
Mr 0 / Mr Crocodile
Win the game with Nico. Robin (Hard mode suggested. Can be unlock on any difficulty setting)
Mr 2 / Bon Clay
After gaining Hina, win the game with Sanji on Hard difficulty setting.
Omu
Win the game with Tony Tony Chopper (Hard mode suggested. Can be unlocked on any difficulty setting)
Play as Ace
Unlock Bon Clay and the third costumes for Luffy, Zorro, Nami, Usopp, Sanji, Chopper and Robin (by completing training mode with them), then successfully complete Event Battle as Smoker under the hard difficulty setting.
Play as Crocodile
Successfully complete Event Battle as Robin.
Play as Orm
Successfully complete Event Battle as Chopper.
Play as Shanks
Unlock Ace, complete Event Battle under the hard difficulty setting as Luffy, Zorro, Nami, Usopp, Sanji, Chopper and Robin, and unlock the Windmill Village and Marine HQ stages. Then, successfully complete Event Battle as Luffy under the hard difficulty setting.
Play as Smoker
Successfully complete Event Battle as Zoro.
Play as Wiper
Successfully complete Event Battle as Luffy.
Portgas.D.Ace
After unlocking all characters above, beat Training mode with all of Luffy's crew and then win the game with Smoker on Hard difficulty setting.
Red Hair Shanks
Once you've gained every other character, win the game on Hard mode using all of Luffy's crew then win the game once more with Luffy on Hard mode.
Shanks' third and fourth costumes
Unlock Luffy's third and fourth costumes, then successfully complete training mode as Shanks.
Smoker
Win the game with Roronoa Zoro (Hard mode suggested. Can be unlocked on any difficulty setting)
Wiper
Win the game with Monkey.D.Luffy (Hard mode suggested. Can be unlocked on any difficulty setting)
Wiper's third costume
Unlock Luffy's and Shanks' third and fourth costumes, then successfully complete training mode as Wiper.