The Grim Adventures of Billy & Mandy Cheats - Wii

Unlock Concept Art Sketch
Hold [1] at the main menu screen, enter [Up], [Up], [Down], [Down], [Left], [Right], [Left], [Right] and then release [1].

Extra costume
Choose the character that you wish to have extra costume and use him/her to clear the Story Mode.

Unlock characters
Unlock Boogey - In Mission Mode Finish Mission 9 in "Tier 4"
Unlock Chicken - Finish Story Mode with all characters on any difficulty with 10 hours of gameplay
Unlock Dracula - In Mission Mode finish Mission 5 in "Tier 3"
Unlock Eris - Finish "Story Mode" once with either Billy, Mandy, Grim, Or Irwin
Unlock Fred Fredburger - Finish "Story Mode" once with either Billy, Mandy, Grim, Or Irwin
Unlock General Skarr - In Mission Mode finish Mission 6 in "Tier 4"
Unlock Hoss Delgado - Finish Mission 9 in Tier 1 in "Mission Mode"
Unlock Jack O'Lantern - In Mission Mode finish Mission 9 in "Tier 2"
Unlock Lord Pain - Finish Mission 9 in Tier 5
Unlock Mogar - Finish "Story Mode" once with either Billy, Mandy, Grim, Or Irwin
Unlock Nergal - Finish "Story Mode" once with either Billy, Mandy, Grim, Or Irwin
Unlock Nergal Jr. - In Mission Mode finish Mission 9 in "Tier 3"
Unlock Nerglings - Finish Story Mode with all characters on any difficulty
Unlock Vikings - Finish Story Mode with all characters and destroy every letter in the credits sequence