Fiona Frost is a major recurring character of the SPY x FAMILY series. She is a spy for WISE who acts under the codename "Nightfall". She works undercover as an assistant to Loid Forger at Berlint General Hospital to provide him with mission details. Fiona is a pale woman of above average height for her age with bobbed hair of a light 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. She is described as beautiful by Yor Forger. Fiona is introduced wearing a duffel coat with a black turtleneck and pants underneath, and her outfit when visiting 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 Twilight to hide her true emotions, Fiona tends to remain emotionless on the surface—even when making a mental confession of love to Loid. Fiona is deeply in love with Loid, to the point of being capable of compromising Operation Strix by visiting the Forger's residence and even telling Sylvia Sherwood and Loid himself to replace Yor and let her take over the role of the Forger wife. Fiona thinks of herself as the superior partner to Loid and wishes to separate him from Yor so they can be together. To others, like her co-workers, Fiona is seen as an unscrupulous and ambitious woman, capable of backstabbing her peers to build a reputation even bigger than Twilight's own. Fiona can act hastily in getting missions done, especially if Loid is involved. For example, when infiltrating the Campbelldon tennis tournament with Loid, Fiona pushed herself to complete their mission and impress him as much as she could. Fiona is also very distrustful of others and is willing to use them as pawns to fulfill her goals, like when she gained the help of a counterfeiter by calling SSS agents on him and subsequently protecting him from them. Despite her methods, Fiona is a proficient spy and capable of partnering with others, like Franky, who she has grown to trust to some extent.