Fionna Campbell

Fionna Campbell, also known as Fionna the Human, is the main protagonist of Adventure Time: Fionna & Cake, and was previously an in-universe fictional character and the gender-swapped version of Finn who were part of part a fanfiction the Ice King supposedly made from a strange red glowing inspiration during his sleep. She and the other gender-swapped characters would occasionally appear in their own segment of the show. Ice King had fantasized about one day making her real and marrying her. It was revealed that Fionna and Cake, along with their world, are real all along. They were actually created by Prismo who at some point became interested in creating his own reality rather than making ones based on others wishes. Since Prismo was not allowed to create his own realities, he hid Fionna and Cake's outside the multiverse and inside the mind of Ice King which led to him creating his fanfiction about it. However, when Ice King was reverted back to Simon Petrikov by GOLB and lost his magic, Fionna and Cake's reality lost magic as well and became a non-magical one. As a part of this, Fionna became an ordinary working human with Cake as her non-magic pet cat.