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+.