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.
Fionna first appears in the episode "Fionna and Cake." They were in the Candy Kingdom's Castle and were helping Prince Gumball decorate for his upcoming party. Cake throws a jelly kinder and Fionna kicks it up onto the ceiling where others are as well. Fionna asks Prince Gumball if they are alive to which he responds "No they can't even talk and covers it's mouth when it tries. He then throws it for Fionna to kick and thanks Fionna. When Cake asks what they are for he replies they are for his Biennial Gumball Ball. Fionna comments that it sounds "large" to which he replies "Yes! So very Large!" He says he wants her to be there which causes her to blush but when he says it is as his pal she stops. The Ice Queen shows up. Dark Clouds cover the candy kingdom and she appears shouting, "The prince shall be mine!" She creates a big mass of Ice that breaks a section of wall of the building to get inside. She comes inside and creates a slush beast that shoots a slush blast pinning Prince Gumball to the wall. She and cake use morning star mode. Cake turns into a mace and Fionna swings her and breaks the slush beast causing it to snow. Ice Queen kidnaps Prince Gumball and disguises herself like him. Fionna believes it is him. He asks her out but she doesn't really understand. Cake says yes for her. Fionna and Cake argue over wether it is a date or not. Fionna brings Cake with her and Prince Gumball(Ice King) brings Lord Monochromicorn. He gives her flowers and Cake catnip. He also gives her a crystal sword. She blushes. Then, they go to the marsh mellowy meadows. Cake tells Fionna to tell Gumball that he's hot to which she refuses. They race, Fionna on Cake and Prince Gumball on Lord Monochromicorn. They end in a field of flowers. He gives her a Pearl Pigmey Skull and gives it to her. Later at sunset he starts singing causing Fionna to blush. As he sings she uses the sword to slice open monsters that explode into Fireworks when sliced open. They finish the song together on the line "I won't let anything in this world keep us apart." They talk and have a good time and at the end he states he wants her to go to the ball with him not as his pal but as his girlfriend causing her to realize it was a date. Cake gets her a dress and purse which she fits her retractable crystal sword into. When she gets to the party she waves to Marshall Lee and follows Prince Gumball to his room. He reveals himself to actually be Ice Queen, as the real Prince Gumball is frozen to the ceiling. She reaches for her retractable sword but it turns into ice binding her hands. She hits it on the ice holding Prince Gumball rescuing him. Cake comes in and swipes Ice Queen's tiara and puts it on. She shoots snow at Ice Queen but Fionna tells her to take it off so she doesn't catch her crazy. Cake takes it off. Prince Gumball is amazed that she saved him and attempts to ask her out to which she refuses. She states that she doesn't really want to date anyone.
- Her name was officially changed from "Fiona" to "Fionna", presumably to make it more similar to Finn's name. The pronunciation, however, has remained unchanged.
- Like Finn had a crush on Princess Bubblegum, it is revealed in many portraits posted that Fionna had a crush on Prince Gumball.
- Fionna has buck teeth like a rabbit.
- 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. 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.