Note: This game is also titled Megami Ibunroku: Devil Survivor.

Successfully complete the game to unlock New Game+ mode. You can select the cleared saved game file to continue.

Successfully complete the indicated task to unlock the corresponding character:

Bishamon, Koumoku, and Zouchou: Take almost any route except for Yuzu; mandatory battles.
Lucifer: Defeat Lucifer in New Game+ mode.
Kresnik and Kudlak: Defeat Kudlak by returning the bag to either Kaido or Mari, and let Mari get the last hit.
Pazuzu: Defeat Kaido.
Yama: Give Kaido Mari's bag, and defeat Keisuke.

Successfully complete the indicated task to unlock the corresponding character icon at the title screen:

Amane: Take Amane's path, and successfully complete the game.
Atsuro: Take Atsuro's path, and successfully complete the game.
Gin: Take Gin's path, and successfully complete the game.
Kaido: Take Kaido's path, and successfully complete the game.
Naoya: Take Naoya's path, and successfully complete the game.
Yuzu: Take Yuzu's path, and successfully complete the game.