Waluigi

" “ Hey! You might be getting better, but nobody cheats better than Waluigi! You got that?! ” ― Waluigi • Mario Golf: Toadstool Tour"

Waluigi is a recurring character in the Mario franchise, who made his debut in Mario Tennis. He is Luigi's self-proclaimed rival, and Wario's partner in crime.

While Wario made his debut as the main antagonist and the final boss of Super Mario Land 2: 6 Golden Coins, Waluigi has never had a major role or even a single appearance in any mainline Mario game outside of spin-offs. Despite this, Waluigi has gained a ton of fame, as one of Nintendo's fan favorite characters.

Why He's Wah!

 * 1) Waluigi is an enigma, even moreso than Wario. His background and beef with Luigi is never given any explanation, he just showed up one day as Wario's tennis partner and has stuck around since. He and Wario aren't even real brothers, according to Charles Martinet, he and Wario were just "two nice, evil guys who found each other." This makes Waluigi fascinating in a way.
 * 2) Waluigi is a classic cartoony rival, embodying many negative traits but played in a comedic way. He's rude, arrogant, sneaky, and scheming. His eccentric personality makes him a fun character, which is exemplified the opening cinematics of Mario Power Tennis and Mario Golf: Toadstood Tour. His wacky personality is also helped by his lanky character design and nasally voice.
 * 3) Mario is often associated with fire, Luigi is often associated with thunder, Wario is often associated with gas, and Waluigi? Waluigi's specialty is plants. In Mario Party 3, Waluigi's Island is filled with Piranha Plants, is seen with a Piranha Plant in game artwork, and uses a Piranha Plant in battle. In the Mario Strikers series, Waluigi has the unique ability to create purple thorny vines. And ever since Mario Party 8, Waluigi now loves to flamboyantly flaunt and pose with roses whenever celebrating his victory.
 * 4) "Orbot: Wario! Have you ever wanted to win the title of Superstar from Mario?! Wario: I'm already a Superhero! Why would I need to worry about some smelly title Mario held? Eggman: Waluigi! Have you ever wanted to win the title of Superstar from Luigi?! Waluigi: Waluigi will hear all of the details, now! ― Dialogue from Mario & Sonic at the London 2012 Olympic Games for Nintendo 3DS showing the difference between Wario and Waluigi's personalities"" “ Just because everyone else is having fun doesn't mean I'm jealous! Not one bit! ” ― Waluigi in denial • Super Mario Party"While to some, Waluigi seems like nothing more than a joke character, there is a rather tragic side to him. Charles Martinet himself has said that the cornerstone of Waluigi's character is self-pity (source). Waluigi believes that everything goes right for everyone except him, and how couldn't he? Mario, Luigi, Toad, Peach, Yoshi, Donkey Kong, and even his own "brother" all have their own adventures, and he's always being left out. The biggest events he ever gets to participate in are just sports, and even then, he's treated as a joke. From the very beginning in his debut in Mario Tennis, his "rivalry" with Luigi is completely one-sided, as Luigi is not intimidated by him in the slightest, and to this day, Waluigi is still only called his "self-proclaimed" rival. It seems that Nintendo is very self-aware of how Waluigi never gets his time to shine, as even his outfit description in Super Mario Odyssey makes reference to how he's never appeared outside of spin-offs  ("A cap with an upside down "L" on it. It's barely used!", "This outfit makes you want to lurk in the shadows, waiting for a turn in the spotlight."). It's very clear that Waluigi is nowhere near as self-confident as Wario is, so his eccentric and flamboyant persona might just be a desperate call for attention. It is for this reason Waluigi can be seen as a sympathetic and even relatable character.
 * 5) Many people also find themselves attracted to Waluigi, due to his flamboyant persona and raw sexual energy. His lanky build, self-pity, and the whole rose thing is enough to make him qualify as a "Tumblr Sexyman".

The Only Bad Quality

 * 1) While Waluigi has all the makings of a great character, from a creative design with a creative name with an entertaining and interesting personality that can make for great interactions with other characters in the series, Waluigi has never appeared in any game outside of sports spin-offs, which is kind of a waste of potential.

Trivia

 * Waluigi's name is a clever portmanteau of Luigi's name and the Japanese word "warui", which translates to "bad" or "evil". There's more, as "warui" can also mean poor, hateful, abominable or inferior - all words that reflect Waluigi's reputation in the Mushroom Kingdom as well as his place compared to Mario Bros. Additionally, Waruiji (with a short i) is an anagram for "Ijiwaru" which means someone who is bad or mean-spirited; further translations are sadistic, malicious and unkind - all fitting descriptions for Walugi’s personality.
 * As video game renders have become more detailed over time, taking a close look at Mario and Luigi's caps, you can very clearly see that their emblems are embroidered onto their caps, but when taking a closer look at Wario's cap, it's made out of noticably cheaper material, and the "W" sticking out makes it look like it was glued onto the cap, and when looking at Waluigi's cap, you can see that the "Γ" emblem on his hat is a cheap sticker with a corner that is peeling off. This bit of trivia is a nice touch that further shows Wario and Waluigi's phony nature.