Beat certain bosses on the right difficulty to unlock them as playable characters in versus mode.

Unlockable	How to Unlock
Aokiji	Beat him him on normal difficulty
Enel	Beat him him on normal difficulty
Final Boss	Beat him in Hard mode
Krieg	Beat him him on normal difficulty
Kuma	Beat him in hard mode
Kuro	Beat him him on normal difficulty
Moria	Beat him him on normal difficulty
Spandam	Beat him him on normal difficulty
Survival mode	Read monument on final island
Wapul	Beat him him on normal difficulty