In SM64: Meet the Luigi, there were many conflicting reports of how Luigi became awkward and cowardly. According to Toad, Luigi was playing with a doll modeled around himself, but accidentally kicked it into a toilet. When he went to retrieve it (getting stuck in the toilet seat in the process), Wario came around and sitting on the toilet seat, proceeded to unknowingly poop on Luigi, scarring the green plumber for life. According to Mario, something bad must have happened to him in his childhood and the scene cut to a flashback revealing a Klepto kidnapping Luigi.
A running gag in the series is that Luigi is abused and humiliated, such as by his own brother. When not being physically attacked, Luigi is often the victim of verbal abuse, including accusations of homosexuality. However, he can also provide comic relief due to being the butt of so many jokes.
Originally, Luigi looked like a re-texture of his brother Mario, just like many other characters in SMG4's videos. However, beginning on March 7, 2016, his appearance changed slightly to better match his current look in the main Nintendo canon, but still as a Super Mario 64 sprite.
He once had two YouTube channels, but the first one was temporarily banned due to Luigi's intro being a ripoff, and the second one was due to Mario's streaking. However this is only in SMG4's canon, and not in real life, as the original channel is still up on YouTube.
Uh... Bro? Are you sure about this?
—Luigi's cowardice gets the best of him.
Luigi is kinder and more understanding than most of the other characters. He is often cheery, enthusiastic, fun, and ready to lend a helping hand. Compared to other SMG4 characters, his personality has not been drastically changed from his Nintendo counterpart. He also relies on common sense, unlike Mario, who usually tries to solve problems with violence.
His main weaknesses are his cowardice (though he was fully willing to rescue his brother twice despite this), in which he gets very nervous when doing something dangerous or immoral, and his easily-exploited kindness, which often results in him being treated worthlessly like a doormat. This version of Luigi is more likely to get scared than the mainstream one, already freaking out upon seeing Goombas (which funnily enough, are the weakest enemies in the Mario franchise) and knocking into stranger's houses. He was also shown in The Toadassass-ination to be stupid as he is seen protecting Koopas when Mario told him to throw them at Toad. Like his mainstream counterpart, Luigi suffers from an inferiority complex, constantly overshadowed by his older brother's popularity and basically being treated as a nobody.
However, when pushed too far beyond the limit, he has been seen to get angry. In these rare instances, he can be very unrelenting and mean, such as when he teamed up with Waluigi, though he usually reverts back to his normal personality when he calms back down. Luigi has also often been accused of being gay. He usually denies this, but he has shown homosexual behavior many times. However, he was attracted to Daisy in The Adventures of Mario and Luigi - Ep. 4, Awkward Weddings but has also watched ***** men on SMG4's computer, so he may be bisexual.
A running gag in SMG4's videos is that Luigi has a tendency to celebrate things too early, which often results in misfortune befalling him. For example, when some of SMG3's forces attacked him, Luigi managed to hold him off. However, even though he was still in hostile territory, he began to celebrate, which resulted in Shrek sitting on him and capturing him.
In some recent videos, Luigi has been shown to be a bit suicidal, offering to sacrifice himself to help his friends in serious moments.
Luigi has committed various crimes in the series, most of the time by accident. He also sometimes gets framed of crimes by his own brother, Mario.
Murder, Assault, and making somebody cry at his ugliness: In A Lost Luigi, Luigi accidentally kicked a man into the sea, threw a ruby into a Toad after the latter lied to him, and when he said "hi" to a nearby man, he freaked out at how ugly Luigi apparently was and fled. This led the policeman in the video to try and arrest him.
Demonic Rituals: In Mario Preschool, Luigi accidentally summoned Satan the Devil (who was revealed to be a good friend of him) while teaching preschoolers about shapes. This led to him being arrested offscreen.
Accidental Mass Murder: In Crime Time, Luigi accidentally murdered a boat of passengers in DK's Jungle Parkway while making a bet with Mario.
Despite his cowardice, Luigi has many abilities that he rarely uses on the field.
Green Missile Attack: Luigi's iconic move from the Super Smash Bros. series, Luigi charges the attack and upon finishing, propels himself like a missile at incredible speed, hitting a nearby opponent and send them crashing into another room. Stupid Smash Bros (where Luigi was brainwashed), SMG4: The Splatfest Incident where Luigi used it to defend Tari, and Final Hours were the only instances where the Green Missile Attack failed (in the former, Mario dodged the attack, and in the latter, during the SMG4 gang's fight with Goku from Dragon Ball, Goku managed to catch said attack without flinching).
Superhuman Strength: Although not as strong like his brother Mario (who even managed to lift a control panel of a train, mind you), Luigi has a great level of strength, which was best demonstrated in High School Mario, where he managed to lift Mario (who according to New Year, New Mario, weighs 14,000 pounds, which equals 7 tons), threw at Bowser a milk carton which was somehow enough to hit the Koopa King and send him flying, and even hitting him and making him crash into a truck.
Superhuman Speed: In Mario's Jackass, Luigi outran an ENTIRE bull stampede. Since bulls usually run 20 miles per hour, this feat is very impressive in itself.
"Death Stare": Originating from the meme of the same name, Luigi gives a person a disapproving and menacing glare as he passes by. In Smexy Soccer, a Boo was utterly horrified by the Death Stare (which is ironic given Luigi's phasmophobia, which means extreme fear of ghosts, and the Boo is a ghost).
Superhuman Agility: Due to his skinniness, Luigi is extremely agile, being able to jump much higher than Mario. More ridiculous is his ability to dodge, as shown in Wild, Wild Mario, where he managed to dodge several gunshots from Wario and Waluigi, without tiring (although he was left traumatized from the constant dodging).
Great Dancing: Luigi is an expert dancer (having shown so many times in Meet the Luigi, much to Lakitu's frustration), having practiced it many times while Mario was away.
Bilingual: Due to his Italian heritage, Luigi knows both the Italian and English languages.
Negative Zone: Also inherited from the Super Smash Bros. series, Luigi initiates a strange dance which causes a green sphere to appear and anyone standing near it to feel dizzy and trip uncontrollably. In Waluigi's Time, Mario states that Luigi can slow down time with this ability.
Swordsmanship: InStupid Luigi's Mansion, while fighting King Boo, Luigi used his Poltergust 3000 as a sword to engage King Boo in a ridiculous sword fight.
Poltergust: The recurring gadget from the Luigi's Mansion series, Luigi uses this vacuum cleaner to suck up any ghost he sees.
Cowardice: Luigi's worst flaw is his extreme cowardice, which he feels whenever doing something immoral or dangerous. In the SMG4 series, the cowardice is so often ridiculous with Luigi freaking out whenever he sees anything that looks scary to him (including Goombas, which are the weakest enemies ever). He only shrugs off this weakness when his loved ones are in danger.
Overconfidence and Recklessness: Sometimes, Luigi can't help but celebrate when something good happens to him, which results in misfortune befalling him.
In "Crystal Funhouse", Luigi is called "Luigi Gaywalker", but it is likely that "Gaywalker" is referring to his role in Mario's Star Wars battle as opposed to being his actual surname. The line "No! I like being Gaywalker!" further supports this possibility.
In "Guards N' Retards: Prisoners" and in "SwagQuest", Luigi is seen driving a kart and making an angry face at the prisoners (to Mario in SwagQuest). This a reference to his "Death Stare" from Mario Kart 8, which is very popular on the Internet. The Music playing while Luigi doing it is "Ridin'" from Chamillionaire.