In order to unlock extra characters, complete the level listed on any difficulty.

Character	How to unlock
Ace	Act 2-4
Akainu	Final Act-2
Brook	Act 1-3
Brook (2 years ago)	Complete Final Act-2, then complete Crew Mission
Buggy	Act 3-5
Chopper	Act 1-3
Chopper (2 years ago)	Complete Final Act-2, then complete Crew Mission
Crocodile	Act 3-3
Eneru	Final Act-1
Franky	Act 1-1
Franky (2 years ago)	Complete Final Act-2, then complete Crew Mission
Garp	Act 4-1
Hancock	Act 2-3
Jimbei	Act 1-2
Kizaru	Final Act-1
Kuma	Act 3-1
Kuzan	Act 2-1
Law	Act 4-2, Must Rescue Whitebeard
Luffy	Available from the start
Luffy (2 years ago)	Complete Final Act-2, then complete Crew Mission
Marco	Act 1-4
Mihawk	Act 3-2
Nami	Prologue-2
Nami (2 years ago)	Complete Final Act-2, then complete Crew Mission
Perona	Act 2-2
Robin	Act 3-4
Robin (2 years ago)	Complete Final Act-2, then complete Crew Mission
Sanji	Act 3-6
Sanji (2 years ago)	Complete Final Act-2, then complete Crew Mission
Smoker	Prologue-2
Teach	Final Act-2
Teach (Double Ability)	Complete all Straw Hat Crew Missions, then Teach's Crew Mission
Usopp	Act 3-4
Usopp (2 years ago)	Complete Final Act-2, then complete Crew Mission
Whitebeard	Act 4-2, Must Rescue Whitebeard
Zoro	3-6
Zoro (2 years ago)	Complete Final Act-2, then complete Crew Mission