Disgaea 2: Dark Hero Days Cheats

Axel/Akutare Mode!
On the start up menu, place your cursor on "New Game" (First option) and input the following buttons: Triangle, Square, X, Triangle Square, X, Circle. Axel/Akutare mode will be unlocked.

Extra Classes
Note that just having the mastery isnt enough you'll then have to petition the Dark Assembly with the appropriate bill to be able to create them.

Archer: Have someone with a level 5 bow mastery
Beast Tamer: Successfully capture a monster
Chaos Soldier: Finish the game once
Druid: Perform a 500+ Geo Combo
Female Ninja: Have someone with a level 5 fist and staff mastery
Gunner: Have someone with a level 5 gun mastery
Heavy Knight: Have someone with a level 3 spear mastery
Magic Knight: Have someone with a level 3 sword and staff mastery
Male Ninja: Have someone with a level 5 fist and sword mastery
Samurai: Have someone with a level 5 sword mastery
Sinner: Someone in your party becomes a criminal

Unlocking Characters and New Character Classes
Asagi: Unlock her map with 33 Felonies, 100 mana, and at any time after the first playthrough, defeat her and pass her bill as a Senator.
Asagi (Humanoid): Raise Asagi's level to 2000 and pass her bill as a senator.
Celestial Host: Clear Akutare mode 3-3 and they will be available to create.
Disgaea Ronin: Have a Disgaea 1 save file. (Regions must match.)
Entei Monster Class: Beat Akutare Mode 2-3 and they will be available to create.
Flonne: Unlock ES8 with 40 hours game time and during Episodes 12-13. Defeat her and she'll join.
Gordon: Unlock Extra Stage 4, and pass his bill after completing it.
Laharl: After obtaining Flonne, pass Flonne's bill as a Senator and beat ES9.
Mao: Unlock his stage after obtaining Champloo and Raspberyl.
Marjoly: Unlock ES7 with 99 Felones, 1500 mana and at any time and defeat her. After, pass her bill.
Mass Produced Kurtis: Beat Extra Stage 4 and you will be able to create them.
Mr Champloo: Unlock his stage with 50 hours game time and Akutare mode clear.
Preire: Unlock ES6 with 66 Felonies, 100 mana and during Episode 1-9 and defeat her. After, pass her bill.
Raspberyl: Unlock her stage after clearing 30 Dark Sun Stages and obtaining Champloo.
Sabre Kitties: Unlocked after clearing Akutare Mode 1-2.
Vyers/Midboss: Unlock ES5 with 20 hours of Game time, 400 mana and in Episode 1-9 and beat his stage. On the next playthrough, get the stage again and pass his bill.
Zetta: Unlock ES11 with 10 Dark Sun Maps cleared and 3000 mana. Clear the map and pass his bill afterwards.

Throw characters through No Entry panels or tall terrain
As long as the obstacle is only one square thick, you can throw through it. Stand next to it and lift a character, then extend your throw through the obstacle. Retract your throw range (toward the thrower) and press X at the same time. There is a "sweet spot" to do it right, but because there is no penalty to missing it (you just get an error sound) you can keep extending and retracting while mashing X until you throw straight through the obstacle.

Unlocking Magichange
To unlock Magichange, beat Axel Mode and start a new game cycle in Adell Mode.

Infinite Magichange turns
You can have infinite turns for Magichange for any character by throwing them back in the base panel. This will work even if the character has already attacked.

Land of Carnage
To unlock the Land of Carnage (a mirror world of every level with super difficult enemies), collect all 16 treasure maps from the different pirates in the Item World.

Land of Carnage: Collect all 16 Treasure Maps

Quickly gain Felonies For Your Party
Normally, only one character can move into the dark court panel in the item world. However, as long as all the characters move onto the panel in a single action, they all get the felony. The easiest way to do this is to have a number of human characters magichange with monsters and have the human characters pick each other up. Then have the bottom character throw the others onto the dark court panel. Keep in mind that only up to 9 characters can be entered into the dark court this way, since the bottom character has to throw the others. Every character will get the felonies, but the dialogue of the prinny judge is determined by the character at the bottom of the stack.