Waluigi

Waluigi, also known as Waluigi Gaywalker, is Luigi's evil cousin and Wario's brother. Waluigi thinks that he is cool and attractive, but he is really ugly and annoying and doesn't know how to write an 'L'.

Personality
While not to the degree his brother Wario is, Waluigi is greedy and will overcome many obstacles for a small personal gain. He is smarter than his brother and tends to be more focused on the task at hand when his cowardice doesn't get the best of him. He is also gay and likes to look at shirtless men.

Wario
Wario and Waluigi are brothers in the SMG4 universe. (it is unknown whether they are brothers in official canon or not) The two are the main protagonists of The Wacky Wario Bros. series, where they go on ill-fated adventures in an attempt to get rich. Neither brother is above harming the other to get an advantage, and they tend to fight a lot.

Mario
Waluigi is a major antagonist to Mario in the bloopers Retarded64: Retard karts 101 and Super Mario Attorney. In the former, Waluigi is the racer that gives Mario the most trouble, and in the latter, he is the opposing side's lawyer.

Luigi
While Waluigi is Luigi's self-proclaimed rival, he has yet to be seen directly antagonizing him in SMG4's bloopers.

Waluigi's Rubber Ducky
Waluigi is shown to have a strong affection for his rubber ducky, dressing him up in a Waluigi hat and squeaking it all the time.

Trivia

 * Waluigi has a fear of apes (specifically Donkey Kong) as seen in The Wacky Wario Bros.: Wario's Treasure Hunting Time.
 * The letter on his hat is actually the Greek letter Gamma. It's unknown if this is intentional or if he's just a retard.
 * Waluigi plays Han Solo in Mushroom wars: that space series?