Fiona was first introduced as an Irish mobster and moonshine swindler, also known by her alias "The Viper", as well as the sister of gang leader and cabaret/brothel owner Finley Flanagan. However, in "Death is a Cabaret", Fiona was revealed to be the infamous serial killer known as "The Scarlet Slayer", having been targeting numerous prostitutes within the Sinner's End section of Concordia. Fiona's method of killing her victims was to slice their abdomens open with a curved knife, leaving a dove feather on the body as a calling card. Prior to "Death is a Cabaret", Fiona's (known) victims were prostitutes Annie and Caitlin.
In "Death is a Cabaret", Fiona's latest victim was Gladys Perrin, knocking her out with chloroform in the bathroom of cabaret Moulin Rose before slicing her stomach open with her signature weapon. When initially questioned by the player and Isaac Bontemps regarding Gladys' murder, Fiona intially cleared herself of suspicion by claiming that she appreciated Gladys' role in improving her illegal alcohol business, doing so as a way to explain a payment she had made to Gladys. She also later scoffed off the detectives' insinuations that she was behind the Scarlet Slayer killings due to her business suffering from the recent Prohibition, citing how all the money she and her brother made went into a shared family coffer and that sabotaging Finley's business would also bring damage to her own.
However, the evidence (including the scar cream Fiona left on her murder weapon) exposed Fiona as Gladys' killer as well as the Scarlet Slayer, and she was confronted by the player and Isaac. After initially denying the charges, the evil Fiona eventually confessed to the killing spree. As for her motivation, Fiona claimed that she killed her victims to "liberate" them and spare them the mistreatment their professions as prostitutes would give them. Repulsed by Fiona's psychotic rant, Isaac arrested Fiona and she was set to be put on trial. But on the way to the courthouse, Fiona managed to escape police custody and flee.
In the preceding case, "Slayer's End", the fugitive Fiona was tracked down to Black Bloom of Dreams, a Chinese opium den, where the player and Maddie O'Malley found Fiona strangled to death. Over the course of the investigation into Fiona's murder, Constable Ramirez discovered the late villainess' hideout in an abandoned train car, which included a wall of photos of Fiona's numerous victims. It was also uncovered that Fiona had been close with prostitute Celia Barnes, who died of syphilis. It was also revealed by Adrienna Brassiere that Fiona had paid for Celia's medical care and stayed by her side until her death, with this most likely being the catalyst for Fiona's killing spree.
Ultimately, Fiona's killer was revealed to be Finley. After Maddie goaded him into confessing to his sister's murder, Finley confessed fully. As he revealed, he had known about Fiona's psychotic ways, but kept giving her work for him and even released her from the Grim Chapel Lunatic Asylum due to her good work. But after learning of Fiona's murderous deeds, Finley realized that his sister had gone off the deep end, with his revelation being confounded by Fiona confronting him at the opium den, enraged at him. Believing doing so would be an act of mercy, Finley killed Fiona by strangling her to death with a curtain rope.
- "Yes, I did kill Gladys and those other girls as well. But I was doing them a favor! (Isaac: "A favor?! You slaughtered them!") Finley slaughtered them when he put them in his employ. Do you have any idea what these girls are put through? Trust me, a little nip now is better than what life had in store for them. (Isaac: "You're claiming you did this out of some twisted compassion for them, not out of spite for your brother?") Of course not! Him or someone else, it made no difference! The only reasonable thing to do was to liberate these girls so they weren't taken advantage of anymore! They are fallen doves. I merely returned their wings to them." (Fiona's delusional justification for her brutal killing spree)