Mr. Cheese from Among Us Logic quickly became a fan favorite thanks to his stupidity, humorous Catchphrase, and status as the resident Butt-Monkey, in spite of only appearing in the first two episodes. He proved popular enough to return in Episode 5, killing his replacement Mr. Egg in the process.
From what you can tell by the amount of views for 6 of the speed paints for Fairy Foxes done by creator StickFreeks herself, Kai, Kiki, and Louie might be in that territory.
If the Emperor Had a Text-to-Speech Device: Rogal Dorn, Fountain of Memes extraordinaire. His portrayal as The Comically Serious, entirely oblivious of the universe's insanity and utterly immune to others' snark (of which there is plenty) has made him beloved by the fanbase, to the point every single thing he says gets memetic status.
Rebecca's crazy theatre teacher, Medusa, despite being a Manipulative Bitch of the highest caliber, grew to be prime Love to Hate material amongst the fandom, with any videos surrounding her being considered some of the most entertaining content Rebecca has put out.
The comments section on "My Teacher Murdered Someone" shows a very vocal love for Rebecca's college friend, Mary, noting that Rebecca had a high probability of being murdered by E, had it not been for Mary being a witness to their conversation. Some fans went so far as to call her the realhero of the story.
Henry the Tooth Fairy from "My Terrible Mouth Accident" quickly became a One-Scene Wonder thanks to how much Refuge in Audacity he provided with his Genre Savviness in knowing that the characters in Rebecca's videos have no mouths. His hilarious performance from Eddie Bowley certainly helped his reputation among fans.
The street performer playing Ready Freddy Fiddlesticks in "Disney Street Performers Pick On Me AGAIN!" garnered a lot of love for his accurate animation representation, his flirtatious attitude towards Rebecca, and his absolute flattery in being drawn by her which he was happy to autograph for her in return.
Caboose was initially created as nothing more than a catalyst for annoying Church, but as time went on and Caboose slowly evolved from a naive but otherwise normal character into the series' biggest cloudcuckoolander, he's managed to become perhaps the series' most popular and beloved character, mainly due to providing some of the funniest moments in the series. For this reason, he's continuously received bigger roles in the plot since.
Agent York and has A.I. partner Delta originally only appeared in the Out of Mind miniseries, with both getting killed off by the end. In spite of that, York's grumpy yet charming attitude and Delta's general intelligence combined with some Not So Above It All moments resulted in the two becoming extremely popular with fans. For this reason, Delta would be brought back as a recurring character in the series, while York would gain a prominent role as a character in The Project Freelancer Saga's flashback storyline.
Both incarnations of C.T., despite both having rather major impacts on the events of the series, don't actually get all that much screen time. In spite of this, the pair's tragic yet beautiful love story has resulted in the two becoming rather well loved by fans.
The generic mook seen surfing the web in the opening scene of Season 10 is surprisingly popular with fans in-spite of how minor of a role he has.
With the exception of Kimball and Doyle, the people of Chorus don't particularly get that much screen time. Despite this, just about all of them have some form of following, the standouts being Katie Jensen (for being adorkable, as well as her hilariously incompetent driving skills) and Dr. Emily Grey (for being a hilarious Mad Doctor with a Genki Girl personality).
Frank, Dylan Andrews' original cameraman, only appears in a single scene in the first episode of Season 15, but is still rather popular with fans for his being a Deadpan Snarker and Sour Supporter towards Andrews.
None of the Blues and Reds get particular focus outside of Temple, but a few of them still manage to be rather popular with fans. Specifically, there's Loco, for being the Blues and Reds' counterpart to Caboose, as well as for turning out to be the Token Good Teammate of the group, and Biff, for his Nice Guy personality, his friendship with Temple, his surprisingly badass moment where he attempts to take on Agent Carolina, and for his tragic Cruel and Unusual Death.
Huggins is well-loved by fans, even by many who weren't particularly favorable of The Shisno Paradox, for her adorkable personality and surprising darker side. Fans were particularly saddened by her death near the end of the Season... then rejoiced when Singularity revealed she survived.
Velvet Scarlatina showed up in a single scene designed to show the audience the prejudice Faunus face on Remnant. She was such a huge hit with the fandom that she was fleshed out into a proper character, put on a team and given a recurring role in the show. Her combat outfit was even given an artists competition where fans could present their ideas, the winner being the design that was eventually used (albeit with a few modifications).
The chainsaw-wielding White Fang heavy who kicked Weiss' ass in "No Brakes" has been getting a surprisingly large amount of mileage.
In Shrapnel, the little girl Reznya saves at the end of KILL CONTRACT: Minions has zero dialogue and maybe around a minute of screen time tops, but fans found Reznya’s interaction with her to be so adorable and heartwarming that people started asking if Reznya would adopt her, leading to the creation of Potato, who uses the same model.
Missi from The Vampair is likable for her charming looks, her vindictive personality, and her singing voice.
