The primary method for expanding your roster is completing the story mode. Most characters and their transformations are tied to defeating specific bosses or completing entire chapters. Chapter Completions : Future Gohan : Clear Chapter 5. : Successfully complete the entire Story Mode.
For those who want immediate access to the 24-character roster (plus transformations), the PPSSPP emulator allows you to import external save data. dragon ball z shin budokai 2 ppsspp unlock all characters