He’s just some guy who randomly showed up one day that became friends with Wario. That makes it even better. Nobody knows where Waluigi came from. He just exists.

whats even better is that Waluigi’s first appearance was in Mario Tennis, meaning that the only reason he showed up in the first place was because he wanted to play tennis with people. nobody had ever heard of him before and he just shows up to play tennis

That’s why Waluigi is never in any of the main games. He only knows Wario and he only wants to play sports. He’s just some guy that met Wario at the tennis court and he hangs with Wario whenever Wario goes out to play sports, and otherwise he minds his own business.

Waluigi probably has his own circle of friends that we’ve never seen. There’s an entire world of Waluigi that we haven’t touched upon simply because he isn’t the adventuring type. Wario phones him up and says “Hey bro, I’m-a gonna go on an adventure to get-a some jewels from a pyramid. You wanna come with?” and Waluigi’s just like “Nah bro, I’m good. Me and my buds are gonna swing by the coffee shop and go to the library today. Hit me up if you wanna go go-karting though!” 

God damn. The mild world of Waluigi.