How do we go about getting Luigi in? Waluigi was created especially to be Luigi's "evil opposite" like Wario is to Mario, but Waluigi hasn't actually been in anything other than the Mario Sports/Kart/Party games. Meanwhile, in addition to Luigi's lone starring role being in Luigi's Mansion, where the Boos were the main enemy, Luigi was also trapped in Big Boo's Haunt in SM64DS, where King Boo was the boss that was guarding him. So now I'm thinking Luigi / King Boo might be a better choice.
...I'm going to go with it. Luigi/King Boo.
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