For Mario Tennis Open on the 3DS, GameFAQs has 42 cheat codes and secrets. That’s hidden, for a total of 17 characters. We show you exactly what steps to take for unlocking all characters for Nintendo’s new 3DS entry in … Join the game you will be able to choose your favorite characters then participate in the tournament. Mario and his friends are back and hosting an extremely fun tennis tournament. Our Mario Tennis Open unlockable characters guide shows you how to unlock all 4 secret characters in Mario Tennis Open.