There are four unlockable bonus characters in Mario Tennis Open for the Nintendo 3DS.
Here are the characters you can unlock, and how to unlock them.
Baby Mario – Complete Super Mario Tennis Special Game 1-3
Baby Peach – Complete the Pro Rings level in the Ring Shot Special Game
Dry Bowser – Complete the Inksplosion level in the Ink Showdown Special Game
Luma – Complete the Superstar level in the Galaxy Rally Special Game
