Originally consisting of SMG4's interpretation of the YouTube Rangers and characters from the Super Mario franchise (such as Mario, Luigi, Bowser and Toad), he later developed his own original characters to add, including but not limited to Fishy Boopkins, Bob Bobowski, Meggy Spletzer, Saiko Bichitaru, Shroomy, Tari, Melony, and the other former members of the Anti-Cast, as well as the less frequent Swagmaster69696969696 and Chris Gordman.
Since 2011, they've been up to hilarious, scary, random, and epic misadventures and shenanigans. However, since 2018, they've also been involved in dramatic and rather emotional and serious adventures in story arcs such as The Waluigi Arc, The Anime Arc and The Genesis Arc.
However, despite their major roles throughout Season 10, majority of the main cast was demoted to minor characters by time Season 11 starts, with the main demotions from major to minor being Tari, Saiko, Fishy Boopkins and Shroomy, due to new characters such as SMG0, SMG1 and SMG2, Shadow Meggy, Boyfriend and Girlfriend, and returning characters such as SMG3, Axol (due to his reletively recurring role in Season 10) and Melony. This ends up bringing other members to being full fledged main characters, such as Meggy (due to her formerly on-going character arc following The Anime Arc) and Melony due to her new human transformation and relationship with Axol.
- SMG4 - The creator of the videos and one of the two main protagonists of the series. He is a partially amnesiac SMG who resides in Peach's Castle with Mario, often making his bloopers (or playing Minecraft), and gets generally annoyed by Mario (who is apparently his assistant), but he likes him nonetheless.
- Mario - The SMG4 universe's avatar and one of the two main protagonists of the series. He is obsessed with spaghetti and incredibly retarded. Almost all of the episodes revolve around him and the trouble that he gets the main cast and himself into.
- Luigi - Mario's younger brother and the deuteragonist of the series. He makes smarter decisions than Mario, but he's still as cowardly as he is in the actual games, if not more so.
- Fishy Boopkins - A Spike who is a huge anime fan due to his obsession with waifus and who owns many Dakimakura pillows. He first appeared in the Seaside Stupidity two-parter episode, where he saved SMG4 and his friends from drowning in the ocean after their cruise ship was wrecked.
- Bob Bobowski - A sentient, stupid and annoying piece of trash who speaks in a text-to-speech voice. Mario and Luigi first met him in the sewers of Ganondorf's castle in SM64 Bloopers: Can Ganondorf come out to play?. He's obsessed with fame, money, and getting the most screen-time in his appearances. He was originally used as a humorous punching bag of a character, but later served as the selfish main antagonist of The Rapper Bob Arc. He has since redeemed himself and become a good friend of Mario's.
- Meggy Spletzer - Formerly a female Inkling, now transformed into a human by The Ink Zuccer 2000, she hails from Inkopolis, but was accidentally captured in a paint bucket in the basement of Peach's Castle and one of the two tritagonists of the series. When Mario accidentally freed her, the two became friends. She competed in Turf Wars with her team, the Splat Squad, until the events of SMG4 Movie: Meggy's Destiny, in which she fulfilled her dream of becoming a Splatfest champion, and she later moved to the Mushroom Kingdom. After she returned from vacation in Port Aurora, she decided on becoming a sports teacher.
- Shroomy - A young anthropomorphic mushroom and the nephew of Doomguy. He was originally a psychopathic demon hunter but had his brain affected when a bite was taken out of his head in an accident in his youth, leading to him becoming the best member of the boy scouts, with over 300,000 badges to his name. He is very resourceful and (tries to be) helpful to everyone, even strangers. His demon hunter persona joined the Anti-Cast in The YouTube Arc, making him serve as his own doppelganger, but after this arc, he saw a decrease in appearances. Nowadays, he makes occasional major appearances.
- Saiko Bichitaru - A beautiful but insane love interest from Kevin's School. Boopkins brought her to life one Valentine's Day with Kamek's wand in order to have a real girlfriend, but due to her being mentally unstable and craving attention, their relationship ended in tragedy. She has since redeemed herself with help from Luigi and become close friends with the gang, though she remains a sociopath to most others. She is an expert guitarist and has developed an obsession with KFC's chicken, and she also possesses a giant hammer.
- Tari - A shy, accident-prone, blue-haired cyborg gamer girl who is obsessed with ducks and lives in an apartment in Onett. She loves video games and is extremely good at them, but outside of them, she hates violence and attempts to avoid it, preferring friendship.
- Melony - Formerly a sentient watermelon and Anti-Cast member who served as Meggy's doppelganger in The YouTube Arc, she transformed into a human girl after a Fierce Deity Mask was accidentally thrown onto her. She is lazy and spends lots of time sleeping, but is able to create a powerful sword at will upon using the power of her mask. She is also a skilled hacker. She was the former best friend/love interest of the late Axol following after. She's the only former member of the Anti-Cast to become the official member of the main cast.
- Toad - A cynical and annoying mushroom person obsessed with candy who is annoyed by Mario's antics and spends most of his time standing in one spot in Peach's Castle. Though he was previously a frequently-appearing character, he has almost exclusively appeared in minor roles and cameos since The Anime Arc, with his only notable major roles since then having been SMG4: Mario vs IT 🤡 and SMG4: Nintendo Gets Deleted, though he still resides in Peach's Castle and is also still the main aide of Peach.
- Bowser - The Koopa King, leader of the Koopa Troop, and one of the two tritagonists of the SMG4 series. In times before, Bowser was always trying to kidnap Princess Peach. Despite them canonically being archenemies, here he is actually one of Mario's friends. He isn't as smart as his Nintendo counterpart and commonly falls victim to his own incompetence. His appearances declined following The Anime Arc, and although he returned for a while in late 2019, his appearances have since seen another decline, and he now only occasionally appears as a member of the gang.
- Peach Toadstool - The ruler of the Mushroom Kingdom and the castle Mario and almost everyone else resides in. She was the prominent female character in early bloopers until Meggy took over that role in late 2017, hence explaining her recent lack of appearances. She has had a few antagonistic appearances in recent years, such as in Part 1 of The Anime Arc and SMG4: Food Wars, however, she is still often acting like a nagging motherly figure to the gang.
- Steve - The default avatar used in the world of Minecraft but a character in the SMG4 universe. He's usually friendly with the rest of the members although insulting Minecraft in any way will result in getting on his bad side. He served as a major recurring character in earlier bloopers but stopped appearing with the crew as of Season 8. After SMG4: War Of The Fat Italians 2018, he mostly appears in Minecraft-themed bloopers such as SMG4: If Mario Was in... Minecraft, SMG4: The Minecraft Plague and SMG4: Steve Fights Everyone.
- Anti-Cast - A group of doppelgängers of SMG4's Gang made by SMG3 to oppose SMG4's Gang in The YouTube Arc . After SMG3's banishment to the Internet Graveyard, all other members have redeemed and became friends afterward.
- JubJub Boopkins - Fishy Boopkins’ baby brother who is often bullied by Mario. He joined the Anti-Cast in S̶M̶G̶4̶: I Can't Believe It's Not SMG4! as a doppelganger to his brother, but as of SMG4: War Of The Fat Italians 2020, he has made up with him. So far, he has only appeared in minor cameos in Season 11.
- Rob the Scarecrow - A sentient scarecrow who lives on a farm with Steve and is obsessed with corn. He is unable to speak on his own, only being able to make squeaking sounds, so he communicates using a hidden radio that uses a text-to-speech voice. He acted as Bob's doppelganger in The YouTube Arc, but has since become friends with him. Like JubJub, he stopped appearing as frequently in Season 11, most likely to continue working on Steve's farm, as shown in SMG4: The Pursuit of Happiness.
- Belle Fontiere - A cyborg gamer girl like Tari. However, she is aggressive and hostile, as opposed to Tari's kindness. She was Tari's doppelganger in The YouTube Arc, but the two later became friends. However, after said arc, she only made a handful of appearances for the rest of Season 10, and she is almost nonexistent in Season 11.
- Whimpu - A meek Japanese boy who is a computer nerd and simps for female Twitch streamers and anime girls, especially Saiko Bichitaru, who he served as the doppelganger of in The YouTube Arc. He appeared more frequently than the other former Anti-Cast members until Melony's transformation into a human, after which his appearances gradually declined.
- Axol † - An Inkopolis-born axolotl and manga artist who became famous while living in Japan and who could make anime characters come to life using a magic pen that he invented, the Inkweaver. He met and assisted the gang in The Anime Arc, after which he moved to the Mushroom Kingdom. At first he was a bright character who was looking for inspiration. Over time, however, it became more of a centerpiece of the gang because it motivated the members. Axol has especially helped Meggy to cope with the death of Desti and also encouraged Meggy not to give up no matter what the situation looks like. His Inkweaver broke and he got a red scar on his right eye and tattered clothes in SMG4: The Internet Graveyard. He later got his Inkweaver fixed in SMG4: If Mario Was In... Anime, which resulted in a series of events at the end of the Genesis Arc that led to him becoming possessed by SMG0 (as 'Ax0l') and eventually sacrificing himself (technically committing assisted suicide) in The Final Farewell, when his love interest Melony killed him at his request in order to prevent SMG0 from eradicating the universe.
- Starman3 - A former friend and ally of SMG4. Due to real-life controversies, he has been removed from SMG4's videos.
- Heavy Squid - The Heavy Weapons Guy with Squidward Tentacles' head. He is a former member of the Splat Squad, Meggy's Splatfest team. He was used as a stand-in for Meggy in The SMG4 Movie: 10 Year Anniversary Special, but it was revealed in the opening advertisement for SMG4: Mario and the VILLAGE that Mario couldn't stand using Heavy Squid as Meggy's stand-in anymore. He has left the gang as of Meggy's return in SMG4: She's Back!.
- Xboxfan1997/Nintendofan1997 - A friend of SMG4 and notable ally in the earlier videos. His appearances have recently been mostly restricted to just being the host of the War of the Fat Italians series.
- FightingMario54321 - Another friend and ally of SMG4. Like X, FightingMario54321 has had fewer appearances since 2015, being relegated to just cameos. He occasionally is seen as a Policeman who seems to hate jaywalking.
- Swagmaster69696969696 - One of the main protagonists of Guards N' Retards along with his partner Chris Gordman. The two have had a variety of jobs over the years, the most notable being their job as guards in a spaghetti manufacturing company named Fresh Spaghetti Tubbie. He's very unintelligent and often causes a lot of the problems in the series, but has been proven to be helpful in rare instances. Although he had played a major role in at least 2 Arc, he is still considered an ally of the SMG4 gang.
- Chris Gordman - One of the main protagonists of Guards N' Retards along with his partner Swagmaster69696969696 who used to work as guards at Fresh Spaghetti Tubbie. He's much more competent and sane than Swag is, though at times Swag has caused him to lose his sanity. He and Swagmaster6969696969 were tertiary antagonists in the first half of The Anime Arc, but became allies in the second half of the arc and have appeared as supporting protagonists since the arc. Although he had played a major role in at least 2 Arc, he is still considered an ally of the SMG4 gang.
- MarioMario54321 - a minor YouTuber and co-leader of the YouTube Rangers. In real life, he has done machinima longer than SMG4 has, but the latter quickly surpassed him in popularity.
- Desti † - An Octoling, Meggy's former rival, and the leader of the Octoposse, a Splatfest team. She appeared as a minor antagonist in the first half of The Anime Arc, but was redeemed and became Meggy's friend in the second half. Tragically, she died at the end of SMG4: World War Mario at the hands of Francis.
- Greg The Alien † - An alien from the planet Aelion who served as its president. After he visited the Mushroom Kingdom in a spaceship, he was captured and sent to Area 51, but he was bust out of the area with the help of Mario. He was grateful for this and stayed at Mario's House until he was accidentally killed by Mario in SMG4: Stupid Mario Arcade.
- Joe Boopkins - Fishy Boopkins' and JubJub Boopkins' caring father. He's basically a giant version of Octodad with Cthulhu's head. With a single strike from one of his tentacles, he crushes anyone who harms his kids or makes them unhappy.
- Sonic the Hedgehog - A super-fast Mobian hedgehog and the main protagonist of his namesake series. He briefly helped Bowser stopped Princess Peach from being engaged by Dr. Eggman in Egg and Peach. He is also the current "waifu" (romantic interest) of both Amy Rose and Swagmaster.
- Dr. Eggman - A rotund mad scientist and Sonic's arch-nemesis. He once helped the group in stopping Crazy Koopa in SM64 Bloopers: Shell Shocked.
- Professor E. Gadd - One of the main characters of Luigi's Mansion and its sequel. He started off as a small character in early videos but has become a recurring character as of 2014. He played a major role in SMG4 Movie: Meggy's Destiny, where he built Ink Bois to help even the playing ground between the human team and the Inklings.
- SMG4's Minion is a clone sister of SMG4 that Mario accidentally created with the cloning machine when trying to clone spaghetti. She is somewhat retarded like Mario and loves hugs.
- MCGustavo is a minor YouTuber. He helped SMG4 catch SMG3 after the latter copied the former's first WOTFI video.
- Ruffman8890, or RM for short, is a minor YouTuber.
- Geofcraze634 is a minor YouTuber. He hasn't made many appearances but he has helped SMG4 out with a few videos in the past. Recently, he has been making Splatoon related videos in his own channel.
- Toadsworth - An elderly Toad who is Princess Peach's steward and who tries to keep her from being kidnapped. Between SMG4: Mario and the Waluigi Apocalypse and SMG4: Mario The Supreme Leader, he only had one minor appearance while his other appearances are just cameos. Before that episode he appeared more often than now.
- Yoshi - A green dinosaur who acts as an ally of the Mario Bros. He is usually depicted as a fruit lover, wallet thief, drug user, weapons dealer, and even a tax fraud criminal in the series. His personality seems to wildly vary with every episode he shows up in.
- Jeeves - A sentient fork who owns several companies and is known to have a very short temper. In recent seasons he has been reduced to a background character.
- New Super Duper Mario Plus Ultra † - A descendant of Mario 500 years in the future where Defective Weegee reign. He teams up with Luigi, Boopkins, and Shroomy, who were frozen for 500 years, to undo the Weegee Uprising. He gets erased from the timeline once the bad future is undone.
- Slenderman - The main character of The Slender Man Mythos and a humanoid abomination who happens to share an odd friendship with Mario since Super Mario 64 Bloopers: My Best Friend Slenderman.
- Old Man Hobo - A recurring side character based on the sprite of the infamous Old Man from Pokemon FireRed and LeafGreen while laying down.
- Susan Wojcicki - CEO of YouTube. After SMG3 had gotten rid of her from stopping his plans, she temporally allies with SMG4's crew to take down SMG3 in The YouTube Arc.
- Siren Head - Another abomination whom Mario befriended. However, he and Slenderman do not get along, as shown in SMG4: SSENMODNAR - 3,826,412 SUB SPECIAL.
- SMG1 and SMG2 - The SMGs of the Toytoy Kingdom who escaped to the Mushroom Kingdom in SMG1's Guardian Pod after the death of their avatar, Spudnick, resulted in the destruction of their universe. They are major protagonists in the Genesis Arc, where they assist SMG4 and his friends in an attempt to get revenge on SMG0 for killing Spudnick.
- SMG3 - Formerly SMG4's former arch-nemesis and a partially amnesiac SMG and the main antagonist of the series. He mainly wants to kill SMG4 and his friends and take over the world, though on some occasions he isn't as harmful as he seems. He had been in an active rivalry with SMG4 since they first met for the first ten seasons. After he was banished to the Internet Graveyard by Susan, he became the ruler of the Internet Graveyard and became an unlikely anti-hero. Later, he would be forced to team up with SMG4 to defeat SMG0 before finally befriending his former enemy. Only after 10 years with the Genesis Arc have SMG3 made an ally.
- Auri Bori - he is Aura Bora and Meggy Spletzer's Partner and the sheriff or deputy of Port Aurora after he sacrifice himself to save Meggy from shot by Benedict after he being revived by phoenix egg.
- Wario - Mario's greedy arch-rival who starred in "The Wacky Wario Bros." episodes for a while alongside Waluigi. He loves money.
- Waluigi - Wario's partner/brother and taco vender. He is also the arch-rival of Luigi who starred in "The Wacky Wario Bros." episodes alongside Wario. After harnessing the power of rejection, he became the main antagonist of the Waluigi Arc.
- Francis † - An anime-obsessed chameleon originating from the game Super Paper Mario, Meggy Spletzer’s arch-nemesis and the main antagonist of Part 2 of the Anime Arc, as well as the main antagonist of the arc overall.
- Anime Secret Service (formerly) - Peach's anime prevention force. They are the Anime Cartel's rival. Despite being officially disbanded since SMG4: Mario Saves Anime, they help SMG4's Gang fight Francis and the Anime Cartel.
- Team Killer Ink - A team of Inklings competing in the final Splatfest event who are racist towards humans. They served as the main antagonists of the computer-animated film Meggy's Destiny.
- SMG0 * - A monstrous virus that consumes Avatars and destroys universes. He is the main antagonist of the Genesis Arc. He is also considered the overarching antagonist of the entire series.
- By Season 11, only Mario, SMG4 and Luigi would remain as the initial members of the gang who have been present since Season 1. Peach Toadstool, Toad, and Bowser would all eventually be retired from the group at some point or another.
- Before Season 8, SMG4's main cast consisted of mainly Super Mario characters. By Season 10, the majority of them have been replaced with original characters such as Saiko Bichitaru, Tari, Meggy Spletzer, and Melony.
- As of Season 10, Fishy Boopkins, Bob Bobowski and Tari are the most frequent members aside from Mario, Luigi, and SMG4 to appear in Season 10.
- Many SM64 Machinimist characters like MarioMario54321, MCGustavo, AustinDawgyDawg, GageDawg, and Pinkolol16, are absent in recent seasons, even in minor cameo appearances.
- Swagmaster and Chris became more prominent recurring characters outside their own series since it eventually merged with the main SMG4 series in 2019 and often times allied with the gang.
- After The Anime Arc Toad became less a prominent character of SMG4’s cast due to his absence in The Anime Arc and Shroomy was his de-facto replacement at the time.
- After The YouTube Arc Toad, Bowser, Shroomy and Axol became less prominent characters of SMG4's cast due to the former Anti-Cast members as their de-facto replacements. Bowser and Axol didn’t appear around with the SMG4 cast in SMG4: Food Wars and SMG4: TEXTURES NOT FOUND. However while Axol started to become of prominence again in Season 11 (until his death), Shroomy and Bowser both continued to not have many major roles, and Toad had almost none.
- Comparisons can be made between them and any team of protagonists, albeit as of late, comparisons to both DC Comics' Justice League and Marvel Comics Avengers have been more prevalent, especially with memes involving Avengers: Infinity War and Avengers: Endgame throughout 2018 and 2019.
- Mario and Luigi are the only members of the gang to not do voice acting.
- However, Luigi is the ONLY one to not speak in full sentences, since sometimes Mario uses voice clips to talk on some occasions.