Waluigi's first appearance was in what game?

Waluigi's first appearance was in the game Mario Tennis 64 for the Nintendo 64 (N64). This game marked his introduction into the Mario series, and he served as a rival character to Luigi. Waluigi's inclusion in Mario Tennis 64 added a new dimension to the game, providing players with a unique and comedic antagonist. Since then, Waluigi has become a popular and recurring character in many Mario spin-off titles, gaining his own fan following and cementing his place in the Nintendo universe.