He is predominantly a Super Mario 64machinimist, as well as Mario's closest ally (rival in the War of the Fat Italians series), the de facto of his gang, and the arch-enemy of SMG3. He also uses material from Minecraft, Garry's Mod, and other media. SMG4 devotes a lot of his time to making these videos, usually uploading a new video every weekend, and sometimes accepts works such as fanart from his channel's viewers and subscribers. In his videos, he often uses quotes, music, and stuff from other famous YouTubers like PewDiePie and Smosh as well as from movies, video games, memes, or TV shows. He is also the oldest original character in the series.
SMG4 heavily practices self-insertion, as he usually appears as a blue and white lookalike of Mario. From his debut, SMG4's character has been passionate and protective of his work; that aside, he seems to play the sane man who is often annoyed or shocked by Mario's crazy, flamboyant idiocy. Despite this, they are friends in most videos, especially in the recent ones.
SMG4, much like any SM64 machinimist, is pretty much identical to Mario, including attire, albeit he differentiates by wearing a blue shirt and hat, and white overalls, but keeps white gloves and brown boots. He's however noted to have different hair, which either differs between silver anime girl hair in pigtails or slick spiky black hair (in reference to his hair in real life).
Well? Do you like it? Isn't it great?
—SMG4 asking Mario about the collab, SMG4: 3 MILLION SUB FAN COLLABORATION!
SMG4 acts as the only sane man at the castle, constantly having to bear the burden of having many idiotic friends. However, he himself is occasionally really dumb as he somehow fell for one of Wario's and Waluigi's scams (they tricked him into buying one million rocks with the promise of making his videos funnier) and even asked for pineapple on a pizza, to which Bowser stated he had a mental disability to the pizzeria. He is very protective of his work, constantly refusing to play games with his friends to make bloopers. In fact, he goes so far to the point of beating his friends up should they touch his things, making him a completely anti-social and lazy person.
He also tricks his friends to go through weird situations, either to teach them life lessons and make them smarter, (High School Mario, Mario University) or for his own benefit (The Mario Hustle). SMG4 is also boastful and stubborn, such as mocking Mario for losing for the 20th time in a row in Super Smash each other in the ass Brothers, claiming he is horrible at gaming, showing him being a sore loser. Despite his flaws, he nevertheless has friendly moments and will sometimes do actions for benevolent purposes.
However, his appearances in earlier episodes up until Season 2 were nothing like his modern days. He had a massive dislike of Mario, was somewhat racist (as shown in his first appearance), and was just as immature as Mario. After Season 2, he became more mature but also quite vain. In Spaghetti Law., he lied to the judge about what caused a man to eat poisoned spaghetti (SMG4 put the poison into the spaghetti to prank Mario) out of fear of being arrested, was a meme dealer, and did other morally questionable choices.
Meme Dealing: In MarioTube 2, it was revealed that SMG4 was an illegal meme dealer, discovered by Luigi. He, however, was not arrested until Mario Preschool.
Using Confiscated Things and Hijacking: In Spaghetti Law., when spaghetti, Mario's favorite food was banned from the Mushroom Kingdom, both SMG4 and Mario tried to bring it back by hijacking a truck filled with the dish.
Stealing and Murder: In 0% of Spaghetti, Mario and SMG4 stole spaghetti from a man and blew his house up, taking the man with it.
Home invasion: In the same episode, he along with Mario, break into this man's house.
Assault: In Mario's Late!, he attacked SMG3 for getting awarded and also for taunting him.
Participating in Illegal Operations: In the Anime Arc, SMG4 defected from the Anime Secret Service and became part of the Anime Cartel, a criminal organization that illegally smuggled anime into the Mushroom Kingdom following its banning. After Peach lifted the ban, SMG4's charges were dropped.
Superhuman Strength: SMG4 is almost as strong as Mario, but cannot physically lift as many things as the plumber can.
Meme Bomb: In 2 MILLION FAN COLLABERATION SPECIAL! (SSENMODNAR), SMG4 created a gigantic ball of energy by gathering energy from internet memes (similar in concept to the Spirit Bomb technique from Dragon Ball). It has enough power to destroy the entire surronding area of Peach's Castle, turning it into a charred wreck.
SMG4 doesn't enjoy listening to the Jonas Brothers.
The song that best describes him the most is Justin Bieber's "Baby".
SMG4 would live underwater in a magical place full of ponies and Teletubbies. This is strange because he said he doesn't like Teletubbies in 0% of Spaghetti, and he is often scared of them in his bloopers.
In-universe, SMG4 doesn't like school.
SMG4 was pretty "demented" as a child, just playing games.
It is implied in several bloopers that SMG4 might actually be gay, or at least bisexual. This is most strongly evidenced by a major scene in Mushroom Wars: That Space Part 2, where SMG4 (playing Obi-Wan Kenobi) not only takes Mario (playing Luke Skywalker), Luigi (playing C3PO) and R2-D2 to a gay bar, but he is approached by a naked man saying that his "butt had gotten bigger since the last time he was there". However, in Ssenmodnar 6.64 (100k Special QNA Edition), he stated he was not gay.
SMG4 has been featured multiple times in the VS show Cartoon Fight Club. Some of his opponents have been SML Mario, Jeffy, ExplodingTNT, and TheOdd1sOut, and he has won against all of them effortlessly.