Beyond Belief: Fact or Fiction is in its entirety on youtube, do what you will with this knowledge
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