Fiona Frost, operating under the code name Nightfall, is a spy for the Western intelligence agency WISE who works undercover as a clerical employee at Berlint General Hospital in the Eastern nation. In this public-facing role, she serves as a direct assistant and colleague to Loid Forger, the alias of the agency's top agent, Twilight. Her introduction into the main narrative occurs when she volunteers to partner with Loid on a mission involving an underground tennis tournament, though her true aim extends far beyond professional cooperation.

On the surface, Fiona presents an almost impenetrable mask of stoicism and icy professionalism, earning her a reputation among colleagues as being cold, ruthless, and excessively ambitious. She rarely displays overt emotion, a characteristic she deliberately cultivated after being trained by Twilight, who emphasized that a spy must never reveal their true feelings. Her surname, Frost, directly reflects this frosty and unreadable exterior. However, this cool facade conceals a deeply passionate and obsessive inner world. Fiona harbors an intense romantic love for her mentor, Loid Forger, and her most fervent desire is to one day become his real wife, fantasizing about a future honeymoon on a southern island.

This hidden motivation colors all her actions, particularly her intense, one-sided rivalry with Yor Forger, Loid's pretend wife for his ongoing Operation Strix. Fiona views Yor as an unworthy and incompetent placeholder for a role she believes she was destined to fill, becoming easily jealous of the domestic rapport Yor shares with Loid. She initially planned to usurp Yor's position by making the operation's wife role seem untenable, but her efforts were thwarted when she realized she could not find a weakness in Yor's seemingly perfect composure. Fiona's internal conflict lies in this tension between her unwavering professional discipline and her overwhelming personal feelings, which sometimes threaten to compromise the very missions she holds sacred.

In her professional role within WISE, Fiona is a highly capable and valued operative who frequently takes on perilous assignments. Her physical prowess and combat skills are considerable, allowing her to single-handedly eliminate enemy agents and overcome significant obstacles. During a mission to acquire a valuable painting at the Campbelldon underground tennis tournament, she demonstrated extraordinary athletic ability and precision, completely dominating opponents who were using performance-enhancing drugs. She is also known for her sharp observational skills and exceptional eyesight, a trait she developed from constantly watching Loid from a distance. While incredibly formidable, her combat abilities appear to be a tier below the superhuman strength of Yor Forger, a fact she begrudgingly confronted in a private tennis match after the tournament concluded.

Fiona's personal development is largely defined by her growing obsession with proving her worth as the superior partner for Loid. Her rigid mindset and lack of empathy can hinder her judgment, as she initially viewed Loid's adopted daughter, Anya, as nothing more than a tool to be trained into a perfect student through a harsh, Spartan regimen. Her deep-seated feelings occasionally surface in moments of extreme pressure or provocation, such as when she unleashed a vicious barrage of tennis smashes against opponents who insulted Loid, claiming her hand simply slipped. In a later mission, when Loid's life was directly threatened, she abandoned her usual restraint altogether and openly expressed her emotions while fiercely defeating the enemy. Despite her best efforts to remain a perfectly unfeeling operative like the man she admires, her actions consistently reveal the passionate and devoted heart she works so tirelessly to hide.