Fiona Frost (フィオナ・フロスト, Fiona Furosuto?) is a spy for WISE who acts under the codename "Nightfall" (〈
Fiona is a pale woman of average height with bobbed hair of a lavender color and a long fringe covering much of the right side of her face, worn swept behind her left ear. She has cold purple eyes framed by light-colored eyelashes, and most often is seen wearing an emotionless expression.
Fiona is introduced wearing a large duffel coat with a black turtleneck and pants underneath, and her outfit during her time at the Forgers' house consists of a black blazer and a low-dip light shirt. Her stud earrings take the shape of WISE's symbol.
After being trained by Loid Forger to hide her true emotions, Fiona tends to remain emotionless on the surface—even when making a mental confession of love to Loid she kept her stone-cold face plastered on. She thinks of herself as the superior partner to Loid and wishes to separate him from Yor Forger so that they can be together.
Fiona was trained under Twilight to become a spy who didn't let their emotions get in the way of the mission. However, the more she works alongside Twilight the more she ends up developing feelings for him. When Twilight was assigned Operation Strix, she wished to be working with him as his wife but at the time she was unavailable because she was doing another operation. Instead, she was placed in Berlint General Hospital to work undercover as an assistant to Twilight so she could provide him with information on missions. The more she watches Twilight, the more she realizes he's developing real feelings for the Forger Family but she finds this trait of his even more endearing. When the two of them are assigned to retrieve the painting "The Lady in the Sun" she lists them as husband and wife for the tennis tournament, showing her feelings for him.
Fiona is able to conceal all of her emotions, making it impossible for anyone to tell what emotion she is feeling. At all times she has a stone-cold look at her face whether she's happy, angry, or even proposing her love.
Fiona has massive physical strength, shown when she took multiple opponents in tennis, and to some degree even managed to rally a shot by Yor.
Twilight served as a mentor during her training as a spy, teaching her to bury her emotions and show no weaknesses. Likely because of this time that she spent with him, Fiona fell in love with Twilight and now has a certain ambition to become his wife in place of Yor, who she deemed unsuitable for such an important role.
Fiona thinks Yor is lacking as Loid's wife and is jealous of the fact that a woman far less skilled than she is is able to play his wife so well, and that she can manage to evoke some genuine positive emotion from Loid when in his company.
- Nightfall wears earrings that have the WISE symbol.