There  is  a  hidden  mode  in Disgaea Portable called Etna mode.
Normally  you  have  to  get  this  mode by going through the game and
reading Etna's diary every chapter, but you can unlock it instantly by
entering  the following code during the title screen, with the pointer
on "New Game." If you're successful, you'll hear Etna's voice.

Cheat code:                                          Effect:
Triangle, Square, X, Triangle Square, X, Circle      Unlock Etna Mode (JP)
Triangle, Square, Circle, Triangle Square, Circle, X Unlock Etna Mode (US)