Cristina, Lincoln's crush from "Making the Case", gained popularity in the fandom despite not even having a single line in her three appearances, often portraying her as a rival to Ronnie Anne or the series' equivalent of Lila Sawyer.
Clyde's parents are popular due to their status as a gay couple. It helps that the show does not attempt to draw attention to this fact, instead focusing on how they are Good Parents to Clyde.
Carol Pingrey, the girl whom Lincoln uses to coerce Lori into being a part of his revised family portrait by threatening to replace her with Carol, due to her looking similar in "Picture Perfect". She appears onscreen during a few seconds, gets a few blink-and-you'll-miss-it mentions afterwards, and doesn't have any line; but she is popular in the fandom. She generally acts as either a rival to Lori (with differences whether she's a sympathetic one or not), Lori's Sitcom Arch-Nemesis whose only joy in life is messing with Lori, a friend of Lincoln (or more than that), a friend of Leni, or even a mix of those portrayals. She was eventually given focus in "Selfie Improvement", in which she's revealed to be Lori's Sitcom Archnemesis since kindergarten, albeit a sympathetic one. They eventually form a friendship at the end.
Kirby from "Out on a Limo", enough for fans to hope he returns in a future episode.
Lincoln's dates in "Dance, Dance Resolution" are popular for the following reasons:
Tabby, the punk girl, became popular long before the episode even aired, as a brief scene with her appeared on a Nickelodeon music video about a month before the episode premiered.
Haiku, the goth girl, gained some recognition, especially once she starts bonding with Clyde.
Giggles for being a legitimately nice girl and, unlike Luan, only teasing Lincoln harmlessly instead of mocking or hurting him with pranks.
Polly Pain for both her design and interesting interactions with Lincoln.
The genderbent versions of the Loud siblings from "One of the Boys" have become very popular even before the episode aired, especially Lars (the male version of Lucy), Loni (the male version of Leni), and Loki (the male version of Lori). It's not uncommon for fans to even ship them with their respective original counterparts.
Maggie, the 13-year-old emo girl from "Funny Business", is pretty popular among fans, so much so that it's common to find her getting shipped with either Luan or Lincoln.
Nikki, Ronnie Anne's tall friend from "City Slickers", became incredibly popular with the fandom.
Upon the airing of "L is For Love", Sam, Luna's crush, became instantly popular with the fandom due to her cute appearance and positive portrayal in the LGBTQ community for later giving the first canon lesbian couple on the show.
Winston, Lola's crush from the above episode, also became popular to a lesser extent, with plenty of fan works featuring him and Lola made shortly after the episode aired. Sadly, unlike Sam, he did not get any more appearances.
Lincoln's lookalike in "Cereal Offender", who is officially known as the Bratty Kid, has some popularity in the fandom. He's speculated to be related to Carol Pingrey, even to the point of being her younger brother to further parallel with Lincoln. Proposed names for him include Linclone and Wilkes.
Pop-Pop is also fairly popular as the resident fun grandpa.
Leni's friend Miguel is starting to gain popularity for not being a gay stereotype and his snarkiness.
The new girl from "White Hare", slowly to the point where she made an actual appearance note only the top of her head was visible in "White Hare" in a later episode, where she was given a name, Stella Zhau, and a redesign.
The rabbit versions of the Loud siblings in the same episode have also become quite popular with the fanbase. It helps that it's a Development Gag to the original concept of the show where the Louds were originally going to be rabbits until it was settled for them to become humans.
Mick Swagger, Luna's idol, thanks in part to being a celebrity who genuinely encourages others to have fun, and just a really cool guy in general. Like Kirby, fans are already hoping that he makes another appearance.
Mollie, a girl from Lincoln's class, is part of this. Although a minor character, the fanbase love her for her friendly rivalry with Lincoln in "Pasture Bedtime."
While she is known to the fandom as "Thicc QT" (take a wild guess why) some refer her as the name "Belle" because of her bell-shaped body. She makes an appearance in the episode "Raw Deal" as a background character during the scene when the Loud family go into the mini mart and she's waiting in line for the bathroom. She had garnered tons of attention from fans due to her design/appearance and has been seen in hundreds of fanart since.
To that end, any number of random, expendable female characters eventually end up with a "QT" title given to them to help make them more identifiable within the fandom. This includes, but is not limited to "Cookie QT" (a Royal Woods Elementary student who sold cookies), "Highlights QT" (Royal Woods Middle student with highlighted hair, who also unofficially known as "Hannah"), "Indian QT" (otherwise known as "Homework Harpy"), "Mall QT" (who Lincoln stood behind in a mall line).
Any of the older sisters' personal friend groups, some of whom are also commonly shipped with each other in fanworks, such as Leni's school friends Jackie and Mandee, Luna's Moon Goats bandmates Sully and Mazzy, and Luan's drama club friends Ruby and Shannon.
