Waluigi

Waluigi is the self-proclaimed rival of Luigi. Envious of Mario & Luigi’s fame, Waluigi schemes to become more popular than the brothers. Contrary to popular belief, Waluigi is not Wario's brother, but rather a persona made up to gain relevance in the Mushroom Kingdom. As such, he likely intentionally designed his own name and appearance to contrast Luigi's.

Birth
Although his birth circumstances are not known, Waluigi is not a star child, as evidenced by his absence from Bowser's plan in Yoshi's Island DS. This supports the statement that he is, indeed, not Wario's brother.

Before Mario Party 3
Fueled by envy, Waluigi spent a long time watching and studying Mario and Luigi from the shadows. During this time he would use his gardening skills to breed and release Piranha Plants in sewers in an attempt to sabotage the Mario Bros. (Link to theory)

Mario Party 3
Waluigi first made himself known to Mario and Luigi during the third Mario Party. Having stolen the Mischief Star Stamp, he introduced himself before defeating Bowser in a fight.

Mario Tennis
After observing them and training, the perfect opportunity presented itself when a Mario Tennis tournament was held. Waluigi entered himself, and claimed to be "Luigi's arch nemesis", teaming up with Wario to steal the brothers' fame.

Physical Appearance
Waluigi is a tall, gaunt, man wearing black overalls, a purple long-sleeved shirt and a purple hat with a yellow upside-down L. He has a large pink nose, thin mustache pointing upwards and blue bags under his eyes.