The world of Wario's lanky partner in crime has been a fixture in the Mario franchise for quite some time, but Waluigi's origins are a bit unconventional.
Unlike almost every other major Mario Tennis on the Nintendo 64 and the Game Boy Color. Because it was a tennis game, each character from the Mario series needed a doubles partner. Mario obviously had Luigi, but Wario didn't have anyone he could team up with. This lead to the creation of Waluigi to serve as a teammate for Wario and also a foil for Luigi. Princess Daisy was also introduced in Mario Tennis to similarly serve as a doubles partner for Princess Peach, who otherwise didn't have a companion.
Since Mario Tennis, Waluigi has appeared in every Mario sports game. Seeing as Camelot Software Planning is largely responsible for the creation Waluigi and nearly every Mario sports game since Mario Tennis, it isn't too surprising they've saved a spot for their own creation. Funny enough, despite being originally created to serve as a pair with Wario, Waluigi has actually appeared in a Mario sports game without his dastardly partner. In Mario Tennis Power Tour for the Game Boy Advance, players can choose to play as Waluigi, but not Wario. Perhaps he was off on his own adventures in Wario World and didn't have time to play tennis.
Super Smash Bros References Waluigi's Origins
The Super Smash Bros roster has become an unofficial home from Mario and many of his friends. Nearly every major member of the Mario franchise is represented on the roster except for one; Waluigi. While this has become a point of contention among fans who desperately want to see Waluigi Smash Ultimate's roster, it seems unlikely seeing as he's already in the game. Although he isn't a playable character, Waluigi is represented in the game as an assist trophy. When a player lucks into getting Waluigi out of an assist trophy, he emerges with a tennis racket in hand and begins stomping enemy players into the ground. This is a clear nod to his Mario Tennis origins and his overall sporty background.
In total, Waluigi has appeared in over 50 different Mario titles since his first appearance in Mario Tennis, and it looks like that list will only continue to grow after appearing in the trailer for Mario Golf: Super Rush. Given how many games he's appeared in, it's a shame Nintendo treats him more like a running joke. The description for Waluigi's outfit in Mario Odyssey states, "This outfit makes you want to lurk in the shadows, waiting for a turn in the spotlight." Nintendo is aware of Waluigi's relative obscurity in the Mario series compared to other characters, but don't seem to have any intentions of changing that fact anytime soon. At least he'll always have Mario Tennis and all the other sports games.