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.
Tom in The Cartoon Show. Aside from the Just Here for Godzilla aspect of him being played by Doug Walker, he also has a lot more scenes than the other side characters, mostly because Doug is the only good actor and the others aren't so bothered.
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.
In-Universe, Lewis' (Linkara's actor and creator) favorite superhero, bar none, is Danny Chase, a Teen Titans character who was generally seen as The Millstone and extremely disliked by the fanbase. Lewis first mentions him in his March of the Titans retrospective in 2013, then again in his Heroes in Crisis review, clarifying that even after seven years, he's still his favorite superhero of all time.
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.
In Dr Glaucomflecken, everyone loves Jonathan, Ophthalmology's "loyal scribe". Not only does he serve to spotlight the importance of auxiliary medical personnel, but he is a Silent Bob and Hyper-Competent Sidekick who can communicate more in a single terse nod than most people can with a sentence. Adding intrigue, there's also the (as yet unresolved) plot arc of the planned uprising of the Jonathans.
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.
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".
Out of the all the friends on Just Dustin's channel, Trent is easily the most popular, due to his impressive strength, loud personality and more often than not winning the majority of the competitions, even when outnumbered.
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.
Matt Santoro's clones Big Shine and Lorrenzo have few appearances, to the point when Matt outright stopped using them as characters, but are popular with the fandom.
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.
SuperMarioLogan used to have Black Yoshi and Mama Luigi. 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! Jeffy has since become the face of the channel, now making him a Breakout Character.
Vester And Friends has Deadpool . Fans Adore him because He is a far more threatening villain than Bowser . He is also liked by the fact that he killed 2 main characters Blue Yoshi and Isabelle and got away with it.
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.
Of course, there's also Rachel's and Jun's cats in Haku, Poki, Nagi, and Pichi, mainly for their very distinct personalities. Out of all of them, Poki seems to be the most popular, due to their love for food and incredibly loud meowing.
A popular Youtube trend for Ensemble Darkhorses is compilation videos of "[Movie] but only when [character] is on screen."
