How I like to think Piranha Plant got into Smash
Toad: Alright, a Smash invitation to... a Mario character?
Waluigi: Well clearly it's meant for me, everyone loves WALUIGI!
Geno: Hey, I've got loads of supporters too!
Toad: Well I have the history, it's clearly meant for me!
Geno: Alright, we'll settle this the old fashioned way... rock paper scissors!
Toad&Waluigi: Agreed!
Rock paper scissors session
Toad: Ha, my rock beats your scissors, Waluigi!
Geno: Well, my paper beats your rock!
Waluigi: My scissors cut your flimsy paper, stupid puppet!
Toad: Look, this clearly isn't working. We need another way to... wait, where'd the invite go?
Piranha Plant Joins the Battle
Toad: Huh... we, uh, should've been watching that invite.
Geno: ...
Waluigi: ...
Toad: ... well... anyone for Rock Paper Scissors?