To play as the following characters, you have to accomplish the corresponding tasks:
Play as Boogey: complete Mission 9 in Tier 4 in Mission mode to
Play as Chicken: complete Story mode with all characters and accumulate 10 hours of game play time
Play as Dracula: complete Mission 5 in Tier 3 in Mission mode
Play as Eris: complete Story mode with Billy, Mandy, Grim, or Irwin
Play as Fred Fredburger: complete Story mode with Billy, Mandy, Grim, or Irwin
Play as General Skarr: complete Mission 6 in Tier 4 in Mission mode
Play as Hoss Delgado: complete Mission 9 in Tier 1 in Mission mode
Play as Jack O'Lantern: complete Mission 9 in Tier 2 in Mission mode
Play as Lord Pain: complete Mission 9 in Tier 5 in Mission mode
Play as Mogar: complete Story mode with Billy, Mandy, Grim, or Irwin
Play as Nergal: complete Story mode with Billy, Mandy, Grim, or Irwin
Play as Nergal Jr.: complete Mission 9 in Tier 3 in Mission mode
Play as Nerglings: complete Story mode with all characters
Play as Vikings: complete Story mode with all characters and destroy every letter in the credits sequence