The Grim Adventures of Billy & Mandy

Clear mission 9 on the 5th tier. If you win the mission, you will hear the narorator say, 'Super secret unlocked'.

You must complete "Story Mode" once with either Billy, Mandy, Grim, Or Irwin.

Costumes (first set) -- Complete the Story mode with any character to unlock an outfit for them.

Characters -- The first 4 unlockable characters can already be played in "VS. Mode". But you have to unlock them in "Story Mode".