Let's be honest, Waluigi is just a manufactured rival. It's been over ten years since he was introduced and he still has yet to appear in a Mario platformer or even a Mario RPG. He's just there for the Mario Kart, Mario Sports, and Mario Party games. Luigi actually has a history with King Boo. First, Luigi took on the Boos in Luigi's Mansion. Then when Luigi was added into Super Mario 64 DS, who was it you had to beat to free him? King Boo. Add in the fact that Luigi's characterization seems to have painted him as somewhat cowardly, and it's only natural that the head ghost would be Luigi's primary antagonist.
Don't support Waluigi. Nominate Luigi/King Boo.
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Nominate The Kid vs. The Guy (I Wanna Be The Guy) for the Greatest Gaming Rivalry Contest!