To get the bosses easily highlight VS Mode press L+R+Y on both controllers, then press START on Controller 1.

Beat the game on Very Hard without losing a battle (yes, it can be done). If you can defeat the old man who appears after you beat Felden Crais, Roomi will pop up after the closing credits roll and tell you to "Highlight VS Mode and press and hold L + R + Y, then press Start.

Bonus-kun (the punching bag), Yacopu (the bunny who morphs into your character), Felden and Warou (the old man) become selectable. Unfortunately, there's no way to save this! So if you turn the game off, you'll have to do it all over again next time you play.