The Grim Adventures of Billy & Mandy Cheats - GameCube
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Unlockable Characters
The first 4 unlockable characters can already be played in "VS. Mode". But you have to unlock them in "Story Mode":
Eris - Beat "Story Mode" once with either Billy, Mandy, Grim or Irwin
Fred Fredburger - Beat "Story Mode" once with either Billy, Mandy, Grim or Irwin
Hoss Delgado - Beat Mission 9 in Tier 1 in "Mission Mode"
Mogar - Beat "Story Mode" once with either Billy, Mandy, Grim or Irwin
Nergal - Beat "Story Mode" once with either Billy, Mandy, Grim or Irwin
Lord Pain - Complete Mission 9 in Tier 5
Nerglings - Beat Story Mode with all characters on any difficulty
Vikings - Beat Story Mode with all characters and destroy every letter in the credits
1st Set Costumes
Complete the Story mode with any character to unlock an outfit for them.