The Mime was well-liked in the preview. Even more in the episode proper thanks to his entertaining animations and resourceful moveset. He is also Thomas Astruc's favoritevillain.
While Lila herself is a Base-Breaking Character, her first akumatized form (Volpina) has plenty of fans due to her design and her high level of cunningness. Even in more modern fanfics, early takes on Volpina as a legitimate heroine and long term character without an akumatization are not unheard of.
Marinette and Adrien's classmates tend not to get much focus outside of episodes where they either get akumatized or get a Miraculous, but several of them have gained a large number of fans:
Nathaniel was already quite popular despite being a background character with barely a couple minutes of screentime. Then "Evillustrator" revealed that he had a one-sided crush on Marinette, and his turn as the Villain of the Week saw him get an ingenious skillset. His popularity has increased dramatically since then. It comes full circle when he becomes the hero Caprikid in "Penalteam" with a power set similar to his supervillain form.
Juleka has a lot of fans, thanks to the heavy implications that she's in a romantic relationship with Rose. She got an even bigger boost in popularity after "Crocoduel" where she becomes the heroine Purple Tigress.
Alix Kubdel is well-loved by fans for her snarky demeanor, her occasionally being the Only Sane Man to the class, and (as of "Reverser") her close friendship with fellow dark horse Nathaniel. "Timetagger" only saw her popularity increase due to the reveal that she'd get the Rabbit Miraculous in the future, acquiring it in the fifth season moments before being Put on the Bus.
Ivan and Mylene are pretty well-liked as well, Ivan for being a Gentle Giant and Mylene for being a Lovable Coward and passionate Soapbox Sadie. Their relationship is considered to be one of the most healthy and heartwarming ones. Them becoming Minotaurox and Polymouse respectively and having major roles in the final episodes of the fifth season have been well-received.
Timetagger, for being quite a fondly remembered Villain of the Week who proved to be one of the most troublesome enemies the heroes have ever faced, as well as his Small Role, Big Impact status of introducing Bunnyx and revealing the Hawk Moth of the future isn't Gabriel. His hammy Rhymes on a Dime nature overall made him a very fun villain to watch.
Prince Ali also gets a lot of fan attention for a character who only appears in one episode. Then again, it does help that the writers clearly never forget about him, since he was mentioned from time to time after his debut, and finally made other reappearances in "Heart Hunter" and "Lies".
Both Markov and his akumatized counterpart, Robostus, the first one for being absolutely adorable and having a sympathetic plight, and the second one for his absolutely brutal but efficient conquest of Paris, near-victory over Ladybug and Cat Noir, and actually turning on Hawk Moth and coming close to murdering him.
Mireille Caquet and especially Aurore Beauréal have many fans, likely due to being central characters in the very first episode. Some are even hoping that the latter will get a Miraculous one day.
For characters that ultimately never made it into the final show, Félix (the original Cat Noir) and the Quantic Kids (Melodie, Mercury, Sparrow, and Kid Mime) have their own surprisingly strong fanbase. Ultimately, Félix ended up appearing in the show in season 3 (with an episode named after him, to boot), while Sparrow ended up debuting in the Miraculous New York special (alongside Mercury as a background character). That being said, there is a sharp line between the concept of Félix and the one on the show, whose popularity is not the same as his original, bad-luck using incarnation.
The Gorilla is well-loved by fans for being just about the only positive adult influence in Adrien's life (with Gabriel and Nathalie being supervillains and the former being emotionally abusive).
Sentibug (the Fan Nickname for the sentimonster Ladybug created in "Ladybug") is beloved by fans and the subject of many an AU where she manages to live past her debut episode.
From the same episode came perhaps the most popular name-dropped character in the entire series: Princess Justice, Marinette's akumatized form. We never see her form, but this has not stopped fans from wanting to see her in person and making their own takes on her potential.
"Miraculous New York" gave us Majestia, Knightowl, Sparrow, and Uncanny Valley, all of whom gained a number of fans pretty much the moment they debuted — the former two for essentially being Superman and Batman as a lesbian couple, the latter two for being adorkable young heroes (Sparrow especially for being a rare example of a major Native American character in animation and eventually wielding a Miraculous of her own).
Initial controversies regarding her design aside, Fei/Renren gained a measure of popularity amongst fans after her debut for being an interesting Anti-Hero character and her dynamic with Marinette.
The Kwamis are well-liked by the fandom due to their adorable/cool appearances, childlike innocence, wanting to use their powers for good, and being the main source of power for the Miraculous. Many fans even made their own Kwamis to match their original Miraculous.
Nooroo is a pretty popular Kwami due to being The Woobie and the main source of Hawk Moth's powers. Many fanfictions like to give Nooroo a good holder who doesn't treat him badly.
Clara Nightingale is also a very well-liked character due to many enjoying her Nice Girl personality and finding her Frightingale persona to be cool.
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