In Six Golden Coins (for GB) Wario has taken over Mario's castle and Mario has to get the 6 golden coins to open the door to the castle. Mario defeats Wario and regains his castle. Wario goes out to look for treasure and a castle beging the Wario serios. Since Wario is consided a "clone" to Mario, Nintendo decided to throw in Waluigi to be Luigi's "clone" in Mario Tennis. I personally like that because now every one has a clone:
Mario-Wario
Luigi-Waluigi
Peach-Daisy
Yoshi-Birdo
Baby Mario-Boswer Jr.
Then there's Bowser and Toad that don't have a clone, although Wart is consided Bowser's clone.