Created for the sole reason of providing Wario with a doubles partner in Mario Tennis on the Nintendo 64 (and allegedly disliked by Mario creator, Shigeru Miyamoto), Waluigi is essentially the "anti-Luigi" for lack of a better description. Like Wario, Waluigi's name is a combination of the Japanese word "bad" (warui) & his good-guy counterpart's name. Although a pun in Japanese, the name Waluigi elicits quite a bit of head scratching in the minds of Western gamers. Waluigi has yet to carve himself out a meaningful niche in the Mario universe, although he has quite a wicked curb-stomp as an Assist Trophy in Super Smash Bros. Brawl, and his pro-wrestling inspired taunts from Super Mario Strikers and Mario Strikers Charged were awesome. Waluigi is a major component of Dance Dance Revolution: Mario Mix and sets the story for that game in motion.
Waluigi is a rather mysterious character. Very little is known about him, although it is somewhat implied that he is the brother of Wario, this has never been proven or stated by Nintendo.