The Baudelaire orphans have encountered quite a few characters during their attempts to escape from Count Olaf's clutches on A Series of Unfortunate Events, from a town full of vindictive Fowl Devotees to an agoraphobic aunt. By and large, whether they have been evil or well-meaning, these people have been largely incompetent and unable to help the trio of genius orphans. But in the belly of the Queegqueg submarine, the Baudelaires seem to have finally met their equal. Fiona Widdershins in the books is also really similar — intelligent, independent, feminist, and with a hidden dark side.
The Baudelaires fled to the Queegqueg hoping to finally find refuge in a VFD-operated submarine. But the person they encounter there is not who they expect. The captain of the sub is a teenage girl, Fiona Widdershins, the stepdaughter of Captain Widdershins, who she says has been missing ever since he answered a distress call from a manatee. The series has given Fiona even more responsibility than she had in the books, as in the novels, Fiona's stepfather was still the captain, and Fiona herself was the submarine's head engineer. But in both her roles as the engineer and as the captain, Fiona is a feminist pioneer, assuring everyone around her that she should not be underestimated due to her gender or her age.
Her intelligence in the show, and in the books, catches the attention of Klaus. They bond over their shared love of literary references, science, and general nerdiness. But while Klaus is blinded by his new crush, Violet becomes increasingly suspicious of the captain. She's secretive, volatile, and unwilling to compromise. Violet tries to convince her brother that Fiona isn't to be trusted, but Klaus just believes that the two girls are too similar and strong-willed to get along.
Fiona takes the Baudelaires on a mission to find the infamous sugar bowl, but before they arrive at the Gorgonian Grotto where it's rumored to be located, Olaf, Esme Squalor, Carmelita Spatts, and Hook-Hands take hold of the Queegqueg with their massive, tentacled submarine. Fiona manages to hide away, but the Baudelaires are caught by Olaf. Desperate to get her hands back on the sugar bowl, Esme orders the Baudelaires to descend into the Grotto to retrieve the sugar bowl.
Luckily, the bowl is already in the safe hands of Quigley Quagmire, but unfortunately for the Baudelaires, the Grotto is filled with Medusoid Mycelium, a poisonous mushroom that kills its victims within an hour. Sunny's helmet fills with the spores, and she becomes infected. When the Baudelaires return to the submarine empty handed and desperate to help Sunny, they find no sympathy from Olaf, who orders Hook Hands to lock them in the brig.
But Hook Hands seems to have a soft spot for the orphans, and when they beg him to help save their sister's life, he allows them to escape to the Queegqueg, where they reunite with Fiona. Luckily, Fiona is a mycologist, and seems to have an encyclopedic knowledge of fungi. Unfortunately, while the VFD was working on a cure for the mushroom's poison, it had not yet been found... until Fiona stumbles upon a hint in one of her books that suggests horseradish might serve as an antidote. They tear the kitchen apart looking for horseradish until Sunny suggests they use wasabi as substitute. With her cooking knowledge coming in handy, Sunny manages to save herself, while Violet traps the spoors in Sunny's helmet and gives it as a gift to Fiona, a culture for her to study.
This turns out to be a massive mistake. The reason for Fiona's secretive nature is revealed when it is uncovered that Hook Hands, actually named Fernald, is Fiona's long lost brother (she thought he had died at the hands of a manatee). In both the book and the series, Fiona decides to join her brother as a member of Count Olaf's troupe, rather than lose him again. In the series, however, Fiona does something even more horrifying— she gives Olaf the Medusoid Mycelium trapped in the helmet.
Though she does let the Baudelaires escape, Fiona refuses to go with them, saying she can't lose her brother again, and in a brief moment of generosity between them, Violet says that she understands. Klaus begs her not to go, but Fiona reminds him that people aren't just either "wicked or noble", but they're like chef salads— a whole bunch of things mixed together. And, just as in the books, she plants a kiss on Klaus before she leaves them.
The ultimate fate of Fiona and Hook Hands is left uncertain both in the novels and in the series. While in the series they are shown one more time in the finale, learning that their stepfather, Captain Widdershins, is alive, in the novels its revealed that the siblings returned to the Fire-Fighting side of the VFD, but that they may have been swallowed up by the Great Unknown, a gigantic sea monster that roams the oceans.
Fiona and Hook Hands represent the first time the Baudelaires seem to be fully confronted with the ambiguities of human nature. Before now, the siblings categorized everyone as either good or evil, on Olaf's side or theirs. After meeting the Widdershins, the Baudelaires begin to realize there is often more overlap between noble and wicked than they might have thought.