Etna Mode (JP)
At Title Menu, put your cursor on New Game (Top Choice). Then press:

X - Y - B - X - Y - B - A

You should hear Etna's voice before it starts up.

Unlockable Etna Mode via New Game +
You opt to do Etna Mode using New Game + feature! When you proceed with Etna Mode using New Game +, enemies will be made stronger in contrast to proceeding by inputting code and it'll also host one of the best levelling spot in the game!

Check out all her entries in her secret chamber before the last chapter and you'll be given the option to do Etna Mode in New Game +.

Pleinar
Simply lose to Mid-boss the first time you meet him. Then on your new clear file, complete the tutorial level and talk to Pleinar. A new option will appear below the usual assembly things. Choose that and she will join at level 1.

Uncovering Etna's Secret Chamber
Unlocking Etna's Chamber: Push the button behind the throne, and another in the skull beside the weapon shop, and check the wall behind the Prinny in the same room as Longinus.

Unlock Prinny Commentary
To unlock the Prinny commentary, start a new game using new game+. The fastest way to get the prinny commentary is to lose to mid-boss to create a new-game+.



Secret Characters
How to unlock the secret playable characters:

Adell Beat "Demon Hall Mirror" stage, in the Cave of Ordeals, 6 times
Marjoly Get rank 9 in the dark assembly, pass the bill "Misterious Seal", and beat all the stages
Plenair After you beat the game, talk to Plenair, and choose the blank option
Priere Get rank 8 in the dark assembly, pass the bill "Altermate Netherworld", and beat all the stages
Rozalin Beat "Demon Hall Mirror" stage, in the Cave of Ordeals, 7 times
Zetta Pass all the enemy bills, then after you get Gordon into your party, beat the first stage in "Stellar Graveyard" again