Waluigi is confirmed to be a returning Assist Trophy and Wario's 2nd Alternate Costume. He is playable in a hack for Brawl, such as MarioKing64DS who modified Waluigi from scratch and made him fully playable as if he was never hacked, but is still a WIP and then he will have his own character slot on the roster rather than replacing Pit once he was finished.
Waluigi has yet to appear in the Wario series.
Nonetheless, he does have many supporters.
Even so, when Super Mario 64 DS came out a group of his fans started a rumor saying that Waluigi would be a playable character.