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

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

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".