Two-Tone is without a doubt the most popular Dalmatian in the series, in fact so popular that fans felt she deserved to be the fourth main pup instead of Spot. She's mainly popular for her unique spot pattern and tomboyish personality. She usually appears in the background in most episodes but has had some speaking parts in a few episodes. Her claim to fame was the episode, "Love 'Em and Flea 'Em" where she played a major role and was in a romantic relationship with Lucky. Also, being voiced by Tara Strong just adds to her popularity.
In a similar fashion, Patch has become quite popular despite looking much differently than he does in the movies. Most fans prefer his movie design because it's cuter, but others prefer his series look as it is closer to Dodie Smith's depiction of him.
Tripod has a fairly large fan base, namely for being a Handicapped Badass. A lot of fans like to pair him up with Cadpig (the general reason being that the two would likely bond over their physical shortcomings and insecurities interpreted by the audience), despite the fact that neither of them conversed with each other at any point in the series.
Mooch, the farm bully, both loved and hated by fans alike. Another character that fans like to pair up with Cadpig since they were both in a romantic relationship in the episode, "My Fair Moochie" (even though Cadpig was only pretending to be his girlfriend.)
Wizzer and Dipstick both seem to have a lot of fans. Often depicted as bullies since they're usually seen hanging out with Mooch. They're also well known for having big parts in the live action films and the video games.
As a one-shot character, Blaze has also been a subject of Cadpig shipping, except in this case, Cadpig did show some genuine interest to him (such as calling him a "hunk" and batting her eyelids to him). It also helps that Blaze is a Nice Guy with amusing acting talent.
Rebecca was a one shot character for the episode, "De Village Elder", but she became a mega fan favorite for all the same reasons as Two-Tone. Although, the fact that she became a love interest of Lucky's caused a lot of fan pairing wars.
Ivy, Cruella's niece, is the single most popular antagonist, even though she only appeared in three episodes.
Amber, Anita's niece, has gotten quite a cult following for a one shot.
The Dalmatians' hero, Thunderbolt, has an impressive fan base of his own outside of the universe. Some have even gone as far to make fan made girlfriends and wives for him.
Jewel is a Dalmatian pup who did not appear once in the series, but she has appeared a lot in fan art with a particular fan depiction. Fans speculate that if Jewel was a character in the series, she'd have a valley-girl-like personality.
Pretty much every Dalmatian pup with a name who didn't appear in the series has a fan base of some kind (e.g. Penny, Pepper, Freckles, Fidget, etc.), Penny especially, considering she had a fairly big role in the animated movie and was originally intended to be one of the main pups in the series (in some countries, she's in place of Cadpig).
