Etna Mode (Japanese version)
At Title Menu, put your cursor on New Game (Top Choice) and then press [X], [Y], [B], [X], [Y], [B], [A]. 
You should hear Etna\'s voice before it starts up.

Unlock 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 +.

Unlock 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.

Extra Classes
How to obtain the hidden character classes in Disgaea DS:

Angel - Female Cleric, Knight and Archer all at level 100 or higher.
Archer - Level 3 or higher in Bow Weapon Mastery.
EDF Soldier - Level 30 or higher Gun Weapon Mastery.
Galaxy Mage - Level a Prism Mage to Level 50.
Galaxy Skull - Level a Prism Skull to Level 50.
Knight - Female Warrior and Female Mage each at level 10 or higher.
Majin - Male Warrior, Brawler, Ninja, Rogue and Scout all at level 200 or higher.
Ninja - Male Fighter and Male Warrior each at level 10 or higher.
Prism Mage - Level a Star Mage to Level 35.
Prism Skull - Level a Star Skull to Level 35.
Rogue - Both Fighter and Warrior, Males or Females, each at level 5 or higher.
Ronin - Female Warrior and Female Fighter each at level 10 or higher.
Scout - 2 Fighters/Warriors, Males or Females, each at level 5 or higher.
Star Mage - Get 1 Fire, Ice and Wind Mage and level all 3 of them to level 5.
Star Skull - Get 1 Fire, Ice and Wind Skull and level all 3 of them to level 5.
Thief - Fighter and warrior both lvl 5.

Prinny Commentary
Complete the game and start a new session using your cleared saved game file.

Adel and Rozelin
Use the following steps to get Adel and Rozelin from the second Disgaea on your team. First, complete the Cave Of Ordeals. 
When this is done the Demon Hall Mirror will be available. 
Do this level four or five times and you will have to fight Adel at level 2,000; 
then Adel at level 2,000 and Rozalin at level 2,500 at the same time. 
When they are defeated they will join your party. Demon Hall Mirror will also become a good leveling up location.

Defeating Item Gods
Once you go into the stage, look at the bonuses for invincibility geo spots. Use your characters to steal all those spots. 
Hopefully you will find an area to pin the Item God in, then just whittle his health down. 
You can use this method to kill the Item God 2 in the Aracadia. 
Depending on your team\'s level, you could be in for a long fight but you cannot lose.