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.

Background
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.

Mario Party 4
Waluigi attended the fourth Mario Party inside the Party Cube.

Mario Kart Arcade GP / Mario Kart Arcade GP 2
Waluigi took part in the Mario Kart Arcade Grand Prix, where there was a cup dedicated to him.

Mario Kart: Double Dash!!
Waluigi took part in the Mario Kart Double Dash Grand Prix, where he once again teamed up with Wario. Their vehicles of choice were the Wario Car and the Waluigi Racer.

Mario Golf: Toadstool Tour
Waluigi made his debut as a golfer in Mario Golf: Toadstool Tour. After seeing Luigi almost get a hole in one, Wario and Waluigi decided to brush up on their golf skills by challenging various other golfers. They started by challenging Yoshi and Koopa Troopa to a round of golf. Waluigi approached the green, but his ball continuously missed the hole. After failing to sink his ball for the fourth time, Wario notified Waluigi that Donkey Kong and Diddy Kong have joined the golfing competition as well, and decided to challenge them next. During this round, Wario hit his ball really far, but he suddenly heard a loud roaring sound shortly after. It is then revealed that Wario's golf ball had given Bowser a black eye, and he began to chase after the duo with his Koopa Clown Car. They managed to lose Bowser by jumping through a Warp Pipe, but little did they know, Bowser's Castle was on the other side. Bowser caught up to them once again, and attempted to throw a Bob-omb at the duo. Just as Bowser was about to throw the Bob-omb, Mario's ball bounced off of Wario and Waluigi, giving them black eyes too, and knocked the Bob-omb off Bowser's hand. The Bob-omb exploded on Wario, Waluigi, and Bowser.

Mario Golf: Advance Tour
Waluigi showed off his golfing skills once again in Mario Golf: Advance Tour.

Mario Power Tennis
Waluigi participated in the Mario Power Tennis tournament, where Wario was once again his doubles partner. They lost to Mario and Luigi in the first round of the tournament. Furious, they decided to vandalize Mario and Luigi's picture on the tournament bracket, which caught the attention of the police. Wario and Waluigi ran from the police for a while until they found a side room in Peach Dome, which is revealed to be Bowser's workout room. Bowser offered to form an alliance with Wario and Waluigi, to which they happily agreed.

Throughout the rest of the tournament, Wario and Waluigi trained in Bowser's workout room. During the final round, the two of them interrupted what was supposed to be a match between Mario & Luigi and Yoshi & Donkey Kong. They pulled out a Bob-omb launcher and attempted to shoot Bob-ombs at the Mario Bros, with Bowser also throwing Bob-ombs from a hot air balloon. After missing several times, Bowser fired a Bullet Bill at Mario. Mario deflected the Bullet Bill with his tennis racket, sending it towards Wario and Waluigi's Bob-omb launcher. The Bob-omb launcher malfunctioned and launched a Bob-omb at Bowser's hot air balloon. Because the balloon's base was filled with Bob-ombs, it caused a huge explosion when it crashed to the ground. Bowser, Wario, and Waluigi all fainted as a result.

Mario Tennis Power Tour
Waluigi also took part in the Mario Tennis: Power Tour tournament, this time without Wario.

Super Mario Strikers
Waluigi participated in the Super Mario Strikers tournament, where he was the captain of his own soccer team. He also assisted in the construction of the Underground field.

Mario Party 5
Waluigi attended the fifth Mario Party that was held in the Dream Depot.

Mario Party DS
Waluigi was one of the eight people to receive an invitation to a free buffet at Bowser's Castle. However, the invitation was actually a trap to get them kidnapped so that Bowser could use his Minimizer to shrink them down to the size of chess pieces. After being thrown to a distant location by Kamek, Waluigi and the rest of the group set off on their journey to Bowser's Castle. Along the way, the eight of them competed for the title of the Superstar.

Mario Party 6
Waluigi attended the sixth Mario Party, where he helped collect Stars to fill the Star Bank and end Brighton and Twila's feud.

Mario Superstar Baseball
Wario received an invitation to Bowser's baseball tournament, and brought Waluigi to play on his team. They managed to advance to the finals, but ended up losing to Mario's team.

Mario Party 7
Waluigi was invited by Toadsworth to go on a cruise for the seventh Mario Party.

Dance Dance Revolution: Mario Mix
In an effort to become the Mushroom Kingdom's best dancer, Waluigi stole the Music Keys from Truffle Tower. He kept one for himself and entrusted Big Blooper, Wario, and Freezie with the other three. Waluigi was forced to forfeit his music key to Mario after losing to him in a dance-off.

Mario Party 8
Waluigi attended the eighth Mario Party, which was held at the Star Carnival.

Mario Kart DS
Waluigi took part in the Mario Kart DS Grand Prix. His vehicles of choice were the Standard WL, the Zipper, and the Gold Mantis.

Mario Strikers Charged
Waluigi's soccer team returned to participate in the Mario Strikers Charged tournament.

Mario Super Sluggers
During Bowser Jr's invasion of the Baseball Kingdom, Waluigi was spotted on a building in Wario City, guarding a treasure chest. However, Wario argued that because he is ballpark's namesake, everything in it belongs to him, including treasure chests. Waluigi agreed to let Wario have the chest, but only if he could complete a baseball challenge that he had set up. Wario won the challenge, and Waluigi was forced to hand over the treasure chest.

Mario Hoops 3-on-3
Waluigi participated in the Mario Hoops 3-on-3 basketball tournament.

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.