Complete the corresponding tasks to unlock the new feature:

Cheerleader A Costume
Complete all cheerleader blocks in the Messy Park Area.

Cheerleader B Costume
Unlock Cheerleader A, then re-defeat all the cheerleaders in Messy Park.

So Long! Round Earth! Cinematic
Lose to the final countdown boss.

Play as Badtz-Maru
Complete the game with the good ending and unlock Badtz-Maru's profile. Start a new game.