Fiona was raised as a volunteer of V.F.D., the Volunteer Fire Department. She had a brother named Fernald, who she got on with, and they got adopted by Captain Widdershins, the owner of the submarine Queequeg who was also a volunteer. Captain Widdershins liked them both, although Fernald betrayed V.F.D. by siding with Count Olaf, one of the leaders of the Schism. Before Fernald abandoned Widdershins, they had violent arguments late at night.
Fiona appears in the eleventh book of the series when she first meets Klaus and the Baudelaires in the submarine itself. She says she is delighted to meet them and is trying to fix a telegram device but she needs Violet's help.
She is much more explanatory and relaxed than Captain Widdershins, whose personal philosophy is "He who hesitates is lost!" and Fiona adds "or she" to the philosophy's plaque. Captain Widdershins basically sums up the situation, tells the Baudelaires to get moving with their jobs on the submarine, and leaves them with their heads spinning. Fiona, on the other hand, is more prepared to delve into the secrets of the situation and exactly why they are out in the submarine and getting to the Hotel Denouement with the infamous sugar bowl.
Fiona takes an immense liking to Klaus and the captain accuses them of flirting. She says there are secrets too terrible to imagine, and she takes a lot of the captain's words to heart. Over dinner, she is of much help - she tells them the Grotto they are headed for is the Gorgonian Grotto, which is named after the mythical Gorgon and has an infamous fungus in it, the deadly Medusoid Mycelium. Fiona is a mycologist, so aside from Klaus, her passion is also for mushrooms and their study.
When they finally reach the Grotto, they have to separate, and Captain Widdershins says a child is needed to retrieve the sugar bowl. Fiona accompanies the Baudelaires on this dangerous mission, and the captain says they will all get certifications for bravery from V.F.D. They are bidden farewell by Captain Widdershins and Phil, the cook, who wave goodbye as Fiona, Klaus, Violet and Sunny prepare to venture into the Grotto.
In the Grotto, they fail to find the sugar bowl, and Fiona bonds with Sunny when she makes a Chinese noodle-Italian sauce meal. Then they venture back out of the Grotto, weary from their unsuccessful mission, only to find the entire submarine is abandoned. Captain Widdershins has abandoned Fiona! Then Klaus tries to calm down Fiona but she bullies him into obeying her. Eventually Count Olaf, in his submarine, comes and takes them all captive, by literally swallowing their submarine into his enormous one.
Inside, Count Olaf boasts that finding the Baudelaires alive is better than finding them dead because now he can kill them all over again, and mistakes Fiona for Sunny grown up. He thinks Sunny's squashed under his foot but he sees its some ridiculous book. Count Olaf recognizes Fiona as the "fungus freak" and says the last time he saw her, he threw thumbtacks into her cradle. He's told by Violet that Sunny is infected with the Medusoid Mycelium and is going to die in an hour unless they give her the cure, but Olaf says how can a family reunion be a terrible thing and shoves the Baudelaires and Fiona in jail.
There, they meet Fernald, their guard, and Fiona is ecstatic at finding her long lost brother. She asks what happened to his hands. Fernald is also delighted at seeing her, and he says how he lost his hands is of no importance right now. Fernald then asks why Fiona joined the Baudelaires, saying there's no wrong side of the Schism. Klaus says yes there is, and Fernald says anyone noble can do something evil and anyone evil can sometimes do something good. Fernald says people are not as simple as V.F.D. makes out.
He tempts Fiona to come with him, and to everyone's surprise, she does. Although the Baudelaires get the cure to Sunny in time, they are shocked to find Fiona joined Olaf's henchmen and is an enemy of V.F.D. Klaus is especially shocked and Fiona betrays him by saying that in their planned food code, Fiona ought to be represented by Klaus favourite food. Then she kisses him on the mouth and leaves to join Olaf. Klaus feels like he was punched instead of kissed.
After her betrayal, Fiona betrayed Olaf with her brother Fernald, and stole the submarine Olaf used. Then Fernald headed for the self-sustaining hot air mobile home that Hector, a volunteer, was carrying the Quagmire triplets in, the two triplets had just met up with their long lost brother, Quigley. Then, Fiona attacked with the Hook Handed Man, and Fernald popped the helium balloons causing the machine to crash.
The Penultimate Peril
Soon the self sustaining hot air mobile home hit the ocean and everyone floundered. However, the Great Unknown, a mysterious submarine which even Olaf was afraid of, appeared underneath everyone. Kit Snicket appeared and yelled at them all to join her and Fiona told Kit to ask Klaus to forgive her for breaking his heart.
Fiona chose to enter the Great Unknown with everyone else and she was never seen again.
Some people believe Fiona only pretended to join count Olaf in order to get herself and the Bauldaires to safety and convince her brother to leave Olaf. This is supported by the fact she immediantly betrayed him.
She is a misunderstood character