Grim Adventures of Billy & Mandy - Alternate costumes

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

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

Play as Fred Fredburger:
Successfully complete Story mode with Billy, Mandy, Grim, or Irwin to unlock Fred Fredburger.

Play as Hoss Delgado:
Successfully complete Mission 9 in Tier 1 in Mission mode to unlock Hoss Delgado.

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

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

Play as Nerglings:
Successfully complete Story mode with all characters to unlock Nerglings.

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

Unlock Boogey:
In Mission Mode finish Mission 9 in "Tier 4".

Unlock Dracula:
In Mission Mode finish Mission 5 in "Tier 3".

Unlock Jack O'Lantern:
In Mission Mode finish Mission 9 in "Tier 2" .

Unlock Lord Pain:
Finish Mission 9 in Tier 5.

Unlock Nergal Jr.
In Mission Mode finish Mission 9 in "Tier 3".