At the title screen, highlight the "New Game" option, then press Triangle, Square, Circle, Triangle Square, Circle, X. If you entered the code correctly, you will hear Etna's voice. In the Japanese version, highlight the "New Game" option at the title screen, then press Triangle, Square, X, Triangle Square, X, Circle. Alternately, successfully complete the game after reading Etna's diary in all chapters. To enter Etna's room, press the two secret buttons in Laharl's Castle. The first button is behind the throne. The second button is inside the skull on the counter in RosenQueen's store. After both buttons are pressed, go to the room where Longinus, Ghoss, Stigma, and Gargo are usually found. Press X in the corner behind Stigma to reveal the door.
Successfully complete the indicated task to unlock the corresponding class:
Angel: Get a Female Cleric, Knight, and Archer all to level 100 or higher.
Archer: Get level 3 or higher in Bow Weapon Mastery.
Celestial Host: Get a Female Cleric, Rune Knight, and Archer all to level 100.
EDF Soilder: Get level 30 or higher in Gun Weapon Mastery.
Galaxy Mage: Get a Prism Mage to level 50.
Galaxy Skull: Get a Prism Skull to level 50.
Knight: Get a Female Warrior and Female Mage both to level 10 or higher.
Majin: Get a Male Warrior, Brawler, Ninja, Rogue, and Scout all to level 200 or higher.
Ninja: Get a Male Fighter and Male Warrior both to level 10 or higher.
Prism Mage: Get a Star Mage to level 35.
Prism Skull: Get a Skull Mage to level 35.
Rogue: Get a Fighter and Warrior (male or female) both to level 5 or higher.
Ronin: Get a Female Warrior and Female Fighter both to level 10 or higher.
Rune Knight: Get a Female Warrior and Female Magician both to level 10.
Samurai: Get a Female Warrior and Female Fighter both to level 20.
Scout: Get two Fighters or Warriors (male or female) both to level 5 or higher.
Star Mage: Get a Fire, Ice, and Wind Mage all to level 5.
Star Skull: Get a Fire, Ice, and Wind Skull all to level 5.
Strider: Get two Warriors or Fighters to level 5.
Thief: Get a Fighter and Warrior both to level 5.