That Dude in the Suede: The curtains, every time fanmail appears, there is always one on the end dedicated to the curtains. Unfortunately, Suede didn't take then with him after he moved (they were his parents', but still...).
The Nostalgia Chick: Nella! It really says something that all videos she's in get dozens of comments about on how awesome she is and how she should have her own show. And the Chick seems to be listening - Nella now shows up in almost every episode.
Atop the Fourth Wall: Linkara. One of many contributors who joined the site in late 2008, and plucked from almost complete obscurity, he quickly rose to become one of the site's biggest draws in only a few short months. Part of this was his unique niche (he is one of two Comic Book reviewers on the site), part of it was his consistent schedule (and at first, he posted more than one video a week, because he had a backlog of video reviews from before he joined the site), and part of it was his Colbert Bump from Spoony, who became a frequent collaborator. As early as January/February 2009, there was talk of him headlining a "comics and animation" section of the site called "Four Color Fantasy" (later renamed "Inked Reality"), but in fact he's so popular he's still listed separately, as an individual, on the site, a distinction shared only by Doug, Lindsay, Brad and Noah.
Devil Boner got immense popularity just from appearing for a few minutes in one review. After Mad Max: Fury Road, A Day in the Limelight episode where he ends up ripping into meninists, he's getting called "best original character in the reboot".
Benny, Hyper's stoic Real Men Wear Pink assassin. Doug has teased Malcolm that, like Devil Boner got popularity from the Estrogen Brigade, people love the character because he looks good in a suit.
Dad is very big with the show's fandom, to the point where they requested he get an episode all to himself. He eventually did get one with a larger focus on him, where he transformed the show into "Computer Entertainment Related Programming" to review Jack Nicklaus' Greatest 18 Holes of Major Championship Golf (misspelled as Jack Nicholson Golf in the episode title and the episode itself) on the NES.
Similarly, Deacon (Dad's actor) has become inordinately popular for a guy who sits in the background and says relatively little, probably thanks in part to some residual Dad-ness.
Takahata101 ended up getting a lot of attention thanks in part to his portrayal of Nappa. LittleKuriboh would even lampshade his popularity by mentioning one of the best way to have a abridged series is having takahata provide a voice in the show.
Nappa from Dragon Ball Z Abridged has easily become one of the series most popular characters, to the point where he had been making occasional appearances as a ghost despite having been completely forgotten in the original work after his death. The Abridged version of the Abridged series lampshades this by giving Nappa the line "I am hilarious and you will quote everything I say!"
Obscurus Lupa often does impressions of characters from the films she reviews. However, while reviewing the Subspecies movies, her Radu impression took on a life of its own, and fans fell in love with her portrayal of him as a dopey, down-on-his-luck guy who's obsessed with boobs. He now has his own Spin-Off, Radu Reviews, and has been upgraded from just being a voice to having Lupa dress up as him, complete with giant Orlok hands.
In the fandom, Andrew Cownden is extremely popular despite not being a member of the core Loading Ready Run crew. The reasons for this are most likely his acting ability, the infrequency with which he appears, and his awesome accent.
During Desert Bus for Hope, Roscoe the duck takes the dark horse title. It just wouldn't be the same without him.
Originally just a mention in the Whateley Universe, Aquerna has become this trope. She's one of the least powerful students at Whateley Academy, and became popular enough that one of the Canon authors is now writing Aquerna stories too.
The Irate Gamer has one in the form of the Wise Sage, even some of the fans who snark and insult the show say he's a redeeming quality, in part probably because he acts slightly better than Bores, another reason why fans probably like him is the fact the person who acts as the Sage doesn't defend Bores as much these days. Interestingly enough, the Sage doesn't appear after the Aladdin review, possibly implying that Bores is either not using him again, or the Sage was grabbing too much attention that he thinks is his. He finally reappeared for a brief cameo in his Cool Spot review, but he didn't get any lines. Chris has seemed to figure out his Darkhorse status and now he's shoehorning in Wise Sage appearances hoping it will make his videos "look better".
Jared Clayton of SOTF-TV is also pretty popular, considering that he isn't even a contestant, just a mentor and has had all of one line (not even onscreen!) in the actual game. This is probably due to the in-character twitter the staff maintains for him, featuring snark galore.
Mr. Kwong, a teacher in v4. It's very unusual for a non-student to get much attention (either in or out of character), but you'd be hard pressed to find a handler that doesn't like him.
The Godmonster of Indian Flats is more well known from these videos than the movie where he's the titular character.
And of course, we have the Excellent Darkhorse himself, Chef Excellence.
The Metamor City podcast has Artax, originally created as an expository engine to explain some of the more complex bits of the world's magic in universe, the fans have often cited him as a favorite character over several of the main characters.
The Queen of Rogues and Robbers was originally supposed to be a one-shot villain in The Questport Chronicles, but has been brought back every year due to her popularity.
The Engineer from True Capitalist Radio. Many of the trolls who call into the show wish that he would host more than Ghost. Some of the trolls have gained popularity after the show's renaming to True Capitalist Radio. The Internet Buttstalker, Tub Guy, Ghetto Capitalist, Horny the Clown and the Ban All Bronies guy/Btown also are popular trolls.
Mabaka! has Maruchi, Rokuro's magic teacher. He was almost certainly designed to be a major prick, yet most readers can't help but love him.
Grass Man from Harry Partridge Bo-Starr and the Champions of Galastrom appears to be a massive fan-favorite.
Due to the nature of the show, a case could be made that anyone besides The Narrator Cecil (and maybe his eventual boyfriend, Carlos the Scientist) from Welcome to Night Vale as this. However, the stand-out by far is Tamika Flynn, the brave twelve-year year old girl who led all of Night Vale's children through the dreaded summer reading program, wears a librarian's severed hand as a necklace, and has formed a militia army of children to fight against Strexcorp.
Cecil: If station management is listening, I, of course, hope we find Tamika Flynn and bring her home safely. I hope she finds you first, that is.
In Worm, a character appears once, in the midst of a crisis, a fat superhero who is named only in passing as Chubster, as the casualties are tallied. He remains unconscious throughout his appearance; the protagonist tries her hand at CPR, fails, and she abandons him to die. Despite his unglamorous nature (or because of it?) and lack of dialogue or action, he is repeatedly referred to in the site's comments section, appears or gets a mention in numerous pieces of fanfiction, and is part of several in-jokes among fans.
Grant Kirkhope on Game Grumps. He appeared as a special guest for a grand total of one episode (though said episode was roughly equivalent in length to about six to seven regular episodes), yet he's often considered an honorary Grump by the fans and often appears in fan art. It helps that he seemed to have an innate sense of the show's sense of humor and meshed incredibly well with Arin and Jon.
Ryan Haywood had been with Achievement Hunter for several years before very rapidly gaining popularity with the fanbase. Some attribute the sudden rise to the development of his "Mad King" persona, which both the other Hunters and the fans seem to relish encouraging.
Random Assault: Mela Firefly. In a poll on the show's Facebook page for which guest hosts fans want to see again, she was the highest-voted.
Rolmex is almost considered the best part of El Rincón de Lonk Plays for being an inefficient villain and appearing in every episode with a different role and plan.
If someone who isnt Aitor Molina or Ganchito appears in AMVs, people will ask for them to return.
Sora has the best lines from the show like "obey your salad".
Tentomon for being voiced by the Professor from Cutrerama.
For Tumblr's 2016 April Fool's prank, they held a parody of the year's elections. Decision 2016 had users vote between four lizards for...something. Each lizard had a basic personality and a blog to use as their platform. The candidates were Ricky, Deborah, Mop, and Wretched Tooth. Mop and Deborah were instant hits with the users, who quickly provided tons of posters, speeches and ads for their favorite candidates. By the end of the event, Ricky had about 6 million votes (though the counter jokingly edited it to 2 votes), Wretched with 8 million, and Mop and Deborah having about double that. Mop ended up winning, with over 14 million votes.
Now one character that actually uses popularity these days is Jeffy. In fact, his page on the SML Wiki is at #1 of most popular pages on the wiki, surpassing other character articles such as Mario and Bowser Junior. He is even regarded as the Creator's Favorite!
Dust, the ultimate Magnificent Bastard with a complex personality and unique motives, quickly became one of the RPG's most popular characters. In fact, he is often hailed as the best-written character in the whole RPG.
Montoya, who served as a Foil to the rather unlikeable Trigger, proved to be much more popular than his partner by acting much more human, which earned him the audience's sympathy.
In The Scathing Atheist Podcast, the crew had a regular segment reviewing terrible religious films, where they brought on their friend Eli Bosnick as a guest. Eli's improvised insults towards the characters and the writing brought such high demand from the fans that they made an entirely separate podcast, God Awful Movies. They spread a crowdfunding campaign on Patreon to fund the show and met their goal in two days.
Ellen Baker became an overnight sensation on Pixiv and Twitter, an especially impressive feat considering she originated from a textbook. Her internet presence first went viral when a Japanese user tweeted pages from his brother's 2016-19 edition of New Horizon (a long-running English language textbook series for junior high classrooms) and commented on its surprisingly animesque illustrations and attractive character designs, most particularly of its blonde English tutor. Being an adorkable, Gorgeous Gaijin, moeteacher, Ellen Baker proceeded to lead an explosion of fanart and R34 images, and many overseas fans professed a desire for similar characters on their own textbooks. Even the original artist took notice, appreciating Ellen's newfound popularity but worrying that her purpose as an instructor would be derailed.
If a Youtuber showcases their pet at any point, odds are that the pet will quickly become more popular than the original Youtuber, with the odds increasing the more often the pet is shown. Examples include Jaiden's pet bird Ari (who actually beat her in a popularity poll she did), and rSlash's pet dog Yugo.