I herd from these websites:

http://www.gamegenie.com/cheats/vboy/mtennis.html

http://www.cheat-database.com/display/singlecheat.php?id=172

http://www.cheatscodesguides.com/vb-cheats/mario-tennis/

That you can play as Wario and Rush (Mega Man’s dog) if you beat the game on singles 5 times on Mario, Luigi and Donkey Kong .Jr. Is this true? I would like to know before I waste my time.

No… you can’t. Those are really old rumors which are untrue.

DogP

I am ashamed I have played the vb since 2007 and only knew about the cheats on the baseball game and since 2010 also of blox 2 🙁

I wish it were true! Wario is my favorite game character and I was sad when I learned he isn’t in Mario’s Tennis 🙁

Rush? What a completely random and odd character to suggest as a secret… I mean no offense to Rush of course, but this isn’t Smash Bros Brawl. 😛

Wario would have been a great addition though. Pretty surprised he wasn’t in it actually to be honest.