Fionna (previously spelled Fiona, and also known as Fionna the Human Girl) is a fictional character and the gender-swapped version of Finn and created by the Ice King in his fan fiction. 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 in the season three episode "Fionna and Cake." She is usually seen in the company of her own stretchy companion: Cake. Also, Fionna's enemy is the Ice Queen.
While usually appearing as black dots, her eyes are seen as blue when enlarged. She wears a rabbit-themed hat (similar to Finn's bear-themed hat) with exposed locks of blonde hair. Unlike Finn, her neck is shown. Her outfit includes a teal blue shirt with elbow-length sleeves, a dark blue skort, and high-knee socks with two thin, light blue horizontal stripes at the top. Fionna is a heavier set than Finn (which Natasha referred to as "chubby cute"). She has a green backpack (similar to Finn's) and Mary Jane shoes.
Revealed on Natasha Allegri's formspring account, the dress she designed for Fionna is based off Princess Serenity's dress from the anime, Sailor Moon (as shown to the right). This may also be the inspiration for Fionna's rabbit-themed hood, as Sailor Moon's protagonist is named "Usagi", which is Japanese for "rabbit." Usagi, or Sailor Moon, also has a talking feline companion named Luna.
A skull that has a hat similar to hers is shown in "Beautopia" shown above the decaying corpse of a cat.
Just as Finn is "all about" things, she claims she is "all about" swords when given the crystal sword.
When admiring the crystal sword, her eyes are shown to be black rather than blue shown later in the episode.
Fionna is strong enough to lift and hold Prince Gumball, similar to how Finn could lift up Princess Bubblegum.
When Fionna is talking to Prince Gumball, she states "not that I'm thinking about butts," which could be a reference to Finn's butt ridden imagination zone in "Rainy Day Daydream."
In Fionna's description on the Cartoon Network website, she is referred to as an alternate version of Finn, rather than a fanfiction character. This may be foreshadowing for a future episode, or an error.