Fionna (previously spelled Fiona and also known as Fionna the Human) is a fictional character and the gender-swapped version of Finn who was created by the Ice King in his fan-fiction. In the real world, she was created by series character designer Natasha Allegri in a series of web comics and drawings. She and the other gender-swapped characters appeared occasionally in their own segment of the show. She is usually seen in the company of Cake, her own stretchy companion and also adoptive sister. Fionna's nemesis is the Ice Queen. She is shown to have an attraction to Prince Gumball and a past with Marshall Lee. Ice King fantasizes about one day making her real and marrying her.

In the spin-off series "Fionna & Cake" it's revealed the Fionna Campbell and Cake the Cat are in fact real people. Specifically, Prismo the Wishmaster wrote the original Fionna & Cake Fan-Fiction and broke cosmic entity law to grant himself a wish to bring his fanfiction to life. This resulted in the creation of a secret unauthorized universe within the multiverse. In order to keep it hidden, Prismo shrunk the universe and uploaded it into the brain of Ice King where nobody would ever find it. This is where, Ice King got the idea for Fionna & Cake and his obsession with trying to bring the characters in his fanfiction to life. Following the Adventure Time Series Finale "Come Along With Me", the magic of the Ice Crown was digested by GOLB reverting Ice King to the mortal Simon Petrikov.This also resulted in the digestion of the magic within Fionna & Cake's Universe, causing everyone to become normal humans and animals with Cake becoming a normal cat incapable of speech.