Mmm, I really like the premise for Waluigi's character:
Wario shows up in the Mario universe and startsbeing a real pest. He bothers Mario at any chance he gets a becomes a veritable rival. Waluigi sees the success of Wario's actions, and so decides to do the same against Luigi. He trains and prepares, gets his nice anit-Luigi costume set up, and finally makes his big debut alongside Wario in Mario Tennis, as if to say, "Here I am, Luigi, your worst enemy! Fear what I've got in store for you!" In response to which Luigi shrugs, shakes his head and says "Luigi not afraid."
And this just drives Waluigi mad.
It's like his whole reason for being hinges upon being hated by Luigi, and Luigi just doesn't care.
On a side note, from what I've heard, the Japanese word for "bad" is "warui" (r and l are exchangable,) and "gi" is informal for "old man." So essentially, Waluigi could mean "bad old man."
Go Moon!
