If you have a NH3 savegame on your memory card everyone except the following characters will be unlocked. This is how you get them.

Unlockable	How to Unlock
Chiyo	Beat RPG Mode
Deidara	Beat him in RPG Mode
Itachi	Beat RPG Mode
Kisame	Beat him in RPG Mode
Sasori	Beat him in RPG Mode
Yondaime	Complete Hero Mode