The Demon Cat from "Dungeon" is amazingly popular despite showing up in only one episode. His approximate knowledge of stuff makes him a good source of Memetic Mutation.
The entireGender Flipped world rivaled the normal one in popularity even before "Fionna and Cake" was made. Special props, however, go to Marshall Lee, who is arguably the most popular one of all, despite his only appearance in that episode being a silent cameo. The episode dedicated to him increased his popularity a billion fold, especially because he's genuinely much more of a mischievous, traditional trickster than Marceline is. It has gotten to the point where they were given a spin-off series on Max, which was very well-received.
Any character that is female, young, and has a fit humanoid body, including Celina, Doctor Princess, Jungle Princess, the Nymphs, the Bikini Babes and others, generally for fanart, shipping and Rule 34.
Jake's dad, Joshua, seems to be rather popular, if only due to how crazy awesome he is.
Before "Jake the Dad" aired, of Jake and Lady Rainicorn's kids, Jake Jr. seemed to be getting the most attention and concern, because the kid lacks a face.
Shoko got plenty of fanart after her debut. Probably because of her sad life and being voiced by Isabelle Fuhrman.
Of the various princesses on the show, the two who have gotten the most attention and subsequent fan art are Breakfast Princess and Frozen Yogurt Princess. The former drew attention due to her unique design, willingness to stand up to both Ice King and LSP, and being voiced by Grey DeLisle, whereas for the latter, while she barely got any screentime, her design and soft voice were viewed as downright Moe, to the point that she gained a cameo appearance in the Series Finale.
Prismo has plenty of fans thanks to his unique design, Odd Friendship with Jake, and being an all-around Nice Guy and Benevolent Genie. As a result, fans were heartbroken when he was killed off by The Lich… and overjoyed when he was brought back to life.
Rosalinen from "Puhoy" is also quite popular, in part to the fact that Finn forgot all about her and their children.
James from "James" seems to have a very loving fan base as well. It helps that, coupled with his funny mannerisms that he's voiced by Brak.
During "Five Short Tables", Ice Queen is revealed to also be writing Gender Flip fan fiction about Fionna and Cake, her fanfic focus on Flynn and Jacques, a gloomy looking boy with bangs covering one eye and wearing formal clothes with his raccoon friend, both got popular instantly. Even Ice President got some fan art and hisGender Flip fanfiction characters, Lynn and Janet also became popular.
