Beat the game to get the 2 player versus mode unlocked. Then, press select on the following characters. Press select again to revert back to the original.

Code - Effect

Hit select on Matilda - Play as Marion Fauna
Hit select on Kanna - Play as Matilda
Hit select on Yoh Asakura - Play as Zeke Asakura
Hit select on Meril - Play as Masked Meril