Disgaea 3: Absence of Justice

Win stage 8-4 with Almaz alone and the battle after.

Level 15 Female Fighter and Level 15 Female Healer.

To unlock the character classes below, fulfill the corresponding requirement.

Berserker: Level 40 Heavy Knight and level 40 Monster Tamer.
Cheerleader (Idol): Level 25 Geo Master and level 25 Healer.
Female Archer: Level 15 Female Fighter and level 15 Female Healer.
Female Ninja: Level 30 Female Monk and level 30 Female Magician.
Female Samurai: Level 35 Female Fighter and level 35 Female Archer.
Geo Master: Level 20 Fist Fighter and level 20 Healer.
Gunner Female: Level 15 Female Thief and level 15 Mage.
Gunner Male: Level 15 Thief and level 15 Male Mage.
Heavy Knight: Level 15 Male Fighter and level 15 Fist Fighter.
Magic Knight: Level 25 Fighter and level 25 Mage.
Majin: Complete the tutorial again on a cleared saved game file.
Male Archer: Level 15 Male Fighter and level 15 Male Healer.
Male Ninja: Level 30 Male Monk and level 30 Male Magician.
Male Samurai: Level 35 Male Fighter and level 35 Male Archer.
Masked Hero: Level 45 Thief and level 45 Gunner.
Monster Tamer: Level 15 Female Fighter and level 15 Fist Fighter.
Ninja Female: Level 30 Female Fist Fighter and level 30 Mage.
Ninja Male: Level 30 Male Fist Fighter and level 30 Mage.
Shaman: Level 25 Geo Master and level 25 Mage.
Star Mage Female: Level 50 female Mage.
Star Mage Male: Level 50 male Mage.

To unlock the characters below, fulfill the corresponding requirement.

Asagi - Clear Extra Map #4
Axel - Clear Extra Map #4
Etna - Clear Extra Map #7
Flonne - Clear Extra Map #7
Laharl - Clear Extra Map #7
Marona - Clear Extra Map #5
Master Big Star - Clear Extra Map #1
Prism Red - Complete Extra Map #3
Salvatore - Complete Extra Map #2

You need to aquire the associated DLC.

If you have not defeated all of the Diez Gentlemen in the side story and obtained the Puppy Paw Stick, there is another way to level up. You must have already completed CH. 4 Almaz the Hero Eternal Teardrop to use this method. Talk to Geoffrey (or Prinny if you are in the side story) and select CH. 4 Almaz the Hero Eternal Teardrop. Summon the character you wish to use and put it in defense. The area is this level has the enemy level up 10% effect. Allow the Warslugs to level up a few times. If you are using a low level character, for example level 1-10, start killing the enemy as soon as they are at level 10s or 20s or they will kill your character. Make sure that you put your character in defense in every turn so that it does not counter attack, and that you have a supporting character to heal you. Once the Warslugs are a few levels above you, start killing them. Begin on the left and make your way to the right. Because of the Geo effect in this area, the enemies will keep leveling up every turn, allowing your character to level up with each kill. Always save the Warslug on the far right for last. Keep far enough so that it cannot attack and just let it level up, but not up to the point where you cannot kill it with the character that you are trying to level. This Warslug can be used to give you a little extra experience and HL boost. Once you are finished, heal yourself at the first aid shop and repeat the process. This is not as fast as other methods, but it does work. As your character gets stronger, let the Warslugs level up more and more each time you enter the stage. You can also vote to make the enemies stronger in homeroom so that you do not have to let the Warslugs level up as much.

Kill more than 99 allies and clear two alt stages by stage 7-6.

Go to homeroom and propose "Watch a New Ending" and clear the stage.

Clear stage 1-9 on 2nd or higher cycle before stage 4-4.

Don't meet any other ending's requirement.