I get what you're saying, Waluigi doesn't have any concrete evidence or official statements that back his case.
And that's totally fair.
But what I'm doing is different, I look for signs, dog whistles, and any new developments for Waluigi in his own series.
Then I bring in the facts, and cross check my findings.
Like this:
Fact #1 Waluigi is a 1st party Nintendo character.
Fact #2 Waluigi is a main Mario character, albeit Second Class or B-Tier.
Fact #3 Waluigi is a Brawl AT, he already has a fully functioning model.
Fact #4 Waluigi is currently the most wanted Nintendo character for Smash.
Fact #5 Waluigi won an official Nintendo Valentine's Day poll by a landslide, securing over 1 million votes over the likes of Mario, Peach, Link, and Isabelle.
Fact #6 Waluigi was directly mentioned by Nintendo on the subject of his absence from Smash Ultimate
4 times, 2 of em were from then President of NoA Reggie Fils-aime.
And so on.
There's also a lot of weird stuff happening, like the Paragon leak, Legacy XP, and the sudden love Waluigi received from Nintendo in other games like Super Mario Party, Mario Kart Tour, etc, without even mentioning his name in the context of Smash unless they were asked directly on the matter.
Point I'm trying to make is:
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This ain't no chair theory.