To get it normally, you'd have to read Etna's secret diary in each chapter.

However, there's another way of getting it:



In the main menu, press the following buttons depending on your control method...

(Playstation Controller): Triangle, Square, Circle, Triangle Square, Circle, X
(X-box Controller): Y, X, B, Y, X, B, A
(PC/default keybinds. If you've changed it, it's the first two options, the fourth option, and the third option on "next page" after the direction binds, going from the top.) I, J, L, I, J, L, K

You should then start the story mode which may look similar to the default story with with ONE key difference. Oh, and this mode is entirely seperate so make sure you don't overwrite your main Disgaea 1 save if you go and save. :)