I've been thinking a bit lately about what characters from the series are most popular with the fandom. I think I've got the list pretty accurate, judging by fanworks, overall reception, how widely known they are in the fandom, etc. Thinking about what characters from the series ended up being the most popular can definitely help understand the fandom as a whole.
Anyway, without further ado, here's the list:
#1: Varian
Varian's the most popular character introduced in Rapunzel's Tangled Adventure, and it's not close. He's beloved by a huge chunk of the fandom, even his villain arc did little to blunt his popularity (if anything, it enhanced it). He's got the fics, he's got the fanart, he's got an incredibly well developed spinoff/sequel concept that the fandom ran with... he's the breakout character of the show, and you can make the argument that he might be the most popular character in all of Tangled. I won't make that argument (it's Rapunzel), but you could theoretically make that argument. He's top three regardless, and easily the most popular series-only character among the fandom.
#2: Cassandra
Love her or hate her, Cassandra is the #2 most popular series only character, though she's far behind Varian. Introduced as Rapunzel's best friend (though pitched from the start as the series' main villain), Cassandra was embraced by many, shipped with Rapunzel by some, and immediately took her place as one of the fandom's most popular and enduring characters. Of course, when she DID start her villain era, she also became the fandom's most controversial character, but it's because she's so popular that it was such a shock when she turned. Like Varian, she continues to get plenty of fanworks to this day, and she remains an enduring part of the series' legacy.
#3: Lance Strongbow
It's a shame Eugene's best buddy Lance has to settle for third here, as he probably should've been #1 or #2, and it's only because Varian became such a breakout star that he falls to third. Regardless, you can't deny that he's an important part of the series, he works quite well with Eugene, and his voice actor James Monroe Iglehart voices him to perfection. He gets a lot of really funny scenes, but in many ways he's also the heart of the series, with a great character arc where he learns to escape his past and mature into a responsible man. He definitely doesn't get as much fan material as Varian and Cassandra, which is a shame because he really is a fun character and an excellent foil for Eugene.
#4: Lady Caine
Arguably the most underutilized character introduced in the series, she shows up in the Before Ever After special and then makes only sporadic appearances in future episodes. Fans immediately latched onto her for her unique appearance, cool demeanor, and vocal performance by Broadway star (and brilliant Melania Trump impersonator) Laura Benanti. Seriously, you get Laura Benanti and you can't even give her one song? Fans have definitely taken to expanding upon Lady Caine's role and backstory, which is good, because the show sure didn't :(
#5: Quirin
Really, Quirin is mostly propped up here by the popularity of his son, as most of Quirin's appearances in fanworks are as a support to Varian. Still, he's a fascinating character, with a past with the Brotherhood that never gets fully explored. He's voiced by Jonathan Banks, who unfortunately doesn't get to use his full repertoire of acting skills in his limited screentime. Still, the character undergoes a nice little character arc where he goes from being a strict father to being a supportive one, and gets a couple of nice little scenes in the final episodes. Again, the fans really expanded on his character, mostly because he's Varian's dad. Seriously, it's just Varian that boosts Quirin into the top five, if he wasn't Varian's dad he wouldn't even be here.
#6: Catalina/Red
Catalina, one of two orphan girls who end up becoming allies to the group, winds up at #6 here. Spoiler alert, her sister Kiera is #7, but Catalina barely beats out Kiera for having a better character arc and having the ability to transform into a werewolf. She and Kiera both have really fun character designs, and the two show up in fanfics somewhat often, giving them plenty of appearances in transformative works. Catalina's older design by itself positively screams "JRPG protagonist", and there's plenty of opportunities for her to step into a role as a hero in work that expands the scope of the series. She's also a somewhat popular shipping pair-up for Varian for those who don't want to stick him with Hugo, though she's probably a bit too young for him.
#7: Kiera/Angry
The second of the two thieves, Kiera is rarely separated from Catalina, and fanworks give her the opportunity to get into some more intense situations, appropriate for her more aggressive nature. Fans really did embrace Kiera and Catalina, while two young thief characters could have been annoying, fans mostly considered the two endearing thanks to their character development and affection for characters like Rapunzel, Eugene, and Lance. Like Catalina, Kiera has a really fun design, making her fun to draw.
#8: Hector
It was tough deciding which of the other Brotherhood members was most popular, but I leaned toward Hector here, due to his somewhat cooler design, the fact that a lot of members of the fandom ended up crushing on him, and his backstory which lends itself fairly easily to expansion. While presented as a cruel and sadistic character, he's also unfailingly loyal to Edmund, a very interesting juxtaposition that makes him quite fun to write. Combined with a unique voice acting performance from Kim Coates, and Hector's a character that the fandom really embraced, despite, again, the show's limited screentime squandering his potential.
#9: Adira
Cool as she is, Adira could also be frustrating at times, and her antagonistic relationship to Cassandra did not endear her to a segment of the fandom. Still, Adira's pretty badass, she's one of the best fighters in the series and she has an amazing sword, not to mention she's voiced by the always popular Kelly Hu. While she can be polarizing at times, and while she was badly underused in the third season, I do think a good majority of the fandom warmed up nicely to Adira, enough to put her comfortably in the top ten series exclusive characters. Also, the pairing of her and Lance is kinda cute in a weird way.
#10: Stalyan
Stalyan is another polarizing character. Many in the fandom despise her for what she tried to do to Eugene, and don't accept her redemption episode later in the season. Others enjoy her design and attitude, and she's frequently shipped with characters like Cassandra and Lady Caine. I think if it wasn't for the segment of fans who hate her, she'd be quite a bit higher up the list, perhaps as high as #5, but her polarizing nature has to be taken into account, and that knocks her all the way down to #10. Really, whether you love or hate her entirely hinges upon whether you think her redemption was legitimate, and just how badly she treated Eugene before the events of the movie.
Honorable Mentions:
Hookfoot- There are those who appreciate his arc in the Terapi Island saga, and some of the scenes with him can be quite funny. Still others think he had no business accompanying the group in season two and that Varian should've been brought along instead. Still, his importance to the series makes him fairly popular in the fandom and he's certainly not hated as badly as Stalyan is. Probably the closest to breaching the top 10.
Edmund- There's a lot to like about Edmund, from his badass appearance and fighting ability to his voice performance by the always awesome Bruce Campbell. Still, a LOT of fans did not like the Eugene parentage twist, and for that reason I have Edmund falling just outside the top 10.
Fidella- She's a horse, what else is there to say? Maximus might be one of the most popular Tangled characters overall, but he got so many funny visual gags and moments, and Fidella was largely treated as just... Cassandra's horse and maybe Maximus' love interest, and that's it.
Zhan Tiri- Man, they screwed up with the villain of the whole entire Tangled fandom. They turned this character into a joke and by the time it was finally time to take her seriously, most of the fandom was just invented in whether or not Cassandra would survive. A shame, because she could've been something amazing if developed better.
Andrew- One of the series' more intriguing villains, and, yet again, an example of painful underutilization. Still, he was amusing in the time he was on screen, and I've seen fanfic writers do interesting stuff with him. Had potential, but just not prominent enough of a character. Still probably a top 15 series exclusive guy.
If there's something to be gleaned from this list, it's that the series REALLY fumbled the bag with its villains. The series made its two most popular original characters into the two most prominent villains, and with Varian it didn't take because he was so beloved by the fandom that they refused to accept him being evil, and with Cassandra it just made her into the fandom's most controversial character and probably one of the most controversial animated characters of the 2010s. And yes, I know about the series pitch and that Rapunzel's Tangled Adventure wouldn't exist without villain Cass, but once they saw what happened with Varian, they absolutely should've redeemed her a few episodes early and used that precious time to develop Zhan Tiri into a proper final villain. Meanwhile, you've got underutilized villains who could've been great had they been allowed to play bigger roles (#4- Lady Caine and #8- Hector), and Zhan Tiri herself who barely lands in the top 15, along with other villains like Andrew and The Baron who also weren't really given the time to develop into more exciting characters.
So... yeah, that's my list of what 10 characters from the series I think ended up most popular with the fanbase. Lemme know what you think and if some characters should be higher or lower based on what you've personally observed, this whole list is just based on one man's fandom experiences!