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.
This was a bad idea...
—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 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.
However, when pushed too far beyond the limit, he has been seen to get angry. On 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.
Like his mainstream counterpart, Luigi suffers from inferiority complex, constantly overshadowed by his older brother's popularity and basically being treated as a nobody.
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.
Luigi has commited 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 (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.
Luigi is fast enough to outrun a bull stampede. Incidentally, he was able to tame the bulls and use them to attack Mario in retaliation for attacking him with them.
He was also able to catch a rocket with his bare hands and throw it back at Waluigi when he attempted to attack him with it. However, he was killed by Toad who was speeding with his Golden Mushroom while he was distracted. he even manged to hold Mario (who according to "New Year,New Mario" weights around 14.000 pounds, which equals to 7 tons)
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.