When you have beaten the game once the following characters can be unlocked by completing the indicated Extra Map.

Unlock Master Big Star:
Beat Extra Map 1

Unlock Salvatore:
Beat Extra Map 2

Unlock Prism Red:
Beat Extra Map 3

Unlock Asagi:
Beat Extra Map 4

Unlock Marona:
Beat Extra Map 5

Unlock Axel:
Beat Extra Map 6

Unlock Laharl, Etna and Flonne:
Beat Extra Map 7

The following list tells you how to unlock ALL 8 endings in the game.

Unlock Normal Ending:
Don't meet any other ending's requirement.

Unlock Human World Ending:
Before you reach stage 7-6 kill more then 99 allies and clear 2 alternate stages.

Unlock Raspberyl's Ending 1:
Lose against Raspberyl on stage 1-5

Unlock Raspberyl's Ending 2:
Lose against Raspberyl on stage 2-1

Unlock Almaz's Ending:
With only Almaz win stage 8-4 and the ensuing battle.

Unlock Laharl's Ending:
Go to Homeroom and propose 'Watch a New Ending' and clear the stage.

Unlock Mao's Ambition Ending:
Before reaching stage 4-4 clear stage 1-9 on 2nd or higher cycle.

Unlock Super Hero Ending:
To defeat the final boss use Mao at Level 500 or higher.

If you want the following character classes in your game they can be obtained by having the indicated characters at the correct level.

Unlock Archer - Male:
Level 15 Male Fighter and level 15 Male Healer.

Unlock Archer - Female:
Level 15 Female Fighter and level 15 Female Healer.

Unlock Ninja - Male:
Level 30 Male Monk and level 30 Male Magician.

Unlock Ninja - Female:
Level 30 Female Monk and level 30 Female Magician.

Unlock Samurai - Male:
Level 35 Male Fighter and level 35 Male Archer.

Unlock Samurai - Female:
Level 35 Female Fighter and level 35 Female Archer.