The Grim Adventures Of Billy And Mandy

Play as Boogey:
Successfully complete Mission 9 in Tier 4 in Mission mode.

Play as Chicken:
Successfully complete Story mode as all characters and accumulate 10 hours of game play.

Play as Clown:
Get 200,000 damage points and play over 100 verses battles.
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Play as Dracula:
Successfully complete Mission 5 in Tier 3 in Mission mode.

Play as Eris:
Successfully complete Story mode as Billy, Mandy, Grim, or Irwin.

Play as Fred Fredburger:
Successfully complete Story mode as Billy, Mandy, Grim, or Irwin

Play as General Skarr:
Successfully complete Mission 6 in Tier 4 in Mission mode.

Play as Hoss Delgado:
Successfully complete Mission 9 in Tier 1 in Mission mode.
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Play as Jack O'Lantern:
Successfully complete Mission 9 in Tier 2 in Mission mode.

Play as Lord Pain:
Successfully complete Mission 9 in Tier 5 in Mission mode.

Play as Mogar:
Successfully complete Story mode as Billy, Mandy, Grim, or Irwin.

Play as Nergal Jr.:
Successfully complete Mission 9 in Tier 3 in Mission mode.

Play as Nergal:
Successfully complete Story mode as Billy, Mandy, Grim, or Irwin.

Play as Nerglings:
Successfully complete Story mode as all characters.

Play as Pumpkin:
Successfully complete Story mode and deal out a total of 100,000 damage points.
Lorna Loera.

Play as Vikings:
Successfully complete Story mode as all characters and destroy every letter in the credits sequence.

Alternate costumes:
Successfully complete Story mode with any character to unlock an alternate costume for that person.

When you complete Story mode with a character that has a costume and you destroy the credits, Mandy will be in a costume.
GH N.

Glitch: The word "Fight!" repeated:
Before the announcer finishes the word "Fight!", quickly pause game play. When the game is resumed, the announcer will say "Fight!" again. This can be repeated as many times as desired.
Jon Bow.