After his very first appearance in "The Spider in the Kinky Boots" preview clip, Travis, the john that Angel Dust sold his services to, received some popularity despite being such a minor one-off character. Those fans were quite happy when he shows up again in "Addict", even if it was mainly for a Funny Background Event. He would later appear in the actual show, again in the background with no lines, in "Masquerade" as a director for one of Valentino's porn videos.
After the "Let's Misbehave" preview clip came out, Charlie's parents, Lucifer and Lilith, exploded in popularity despite only appearing in background portraits. This is especially noteworthy for Lucifer as this clip was essentially his first appearance in any sort of form as far as it's concerned. Lilith on the other hand, had previously made an appearance in a background portrait in the first trailer but it was only after this clip that she really garnered some real popularity.
As of the "Addict" music video, Angel Dust's pet pig Fat Nuggets. Everyone loves Fat Nuggets.
Dia and Summer, the succubus and the cat girl who were acting as Valentino’s accessories in the “Addict” video have also gained some popularity. Partly because they make a cute couple (assuming them kissing in the limo was because they wanted to and not to please Valentino, that is).
The sheep girl from Alastor’s comic has gained tons of fanart and attention from the fandom thanks to her adorable design and personality, despite only showing up in a few pages.
Cherri Bomb appears for around two or so minutes in the pilot and has a bunch of fans. Most of it is for her design, her spunky attitude and her clearly genuine friendship with Angel Dust. The fact she was willing to take on Valentino for him only increased her popularity. Many fans are hoping that she becomes a major character on the show and even to become a guest at the hotel, which she does at the end of season 1.
Crymini, a character who is still in development, only appeared in the pilot for a few seconds without a speaking role, and yet, she already has tons of fans. Her cool design is most likely the reason behind her popularity. Many are hoping to see her become a major character on the show and even to become a guest at the hotel.
Zeezi, who was a background character in the Pilot and had one line in "Scrambled Eggs" (with her name only being known due to the Closed Captions), immediately received love from many fans (likely thanks to her gigantic dinosaur-like design).
Alastor's nun outfit only appeared for two seconds as a gag in "Dad Beat Dad", but fans latched on to it and thought it was hilarious. There's even cosplay of it!
In a similar case, Vox's bishop outfit has also received plenty of love and fanart, despite appearing briefly in "Stayed Gone", mostly for serving as nun!Alastor's Foil.
Zestial appears in one episode in Season 1 and is mainly used as a plot device character to start Carmilla and Velvette's Quarreling Song "Respectless" that discusses the decapitated angel as well as there to comfort Carmilla during "Whatever It Takes". However, his uniquely intimidating and mysterious design, enticing deep voice and manner of speaking, and his surprising geniality made him an immediate fan favorite.
Eve, the second wife of Adam and the one who accepted the Forbidden Fruit, even though she hasn't appeared outside of a flashback as a silhouette, became very popular thanks to the fascinating enigma of what happened to her afterward. A common theory is that she's Roo, a confirmed villain for later in the series.
, the john that Angel Dust sold his services to, received some popularity despite being such a minor one-off character. Those fans were quite happy when he shows up again in "Addict", even if it was mainly for a Funny Background Event. He would later appear in the actual show, again in the background with no lines, in "Masquerade" as a director for one of Valentino's porn videos.