"There it is. That's the Twilight who won my heart. The Twilight I can't stop chasing. The Twilight I want at my side. The Twilight who turned my respect into love. The Twilight whose dreams became my dreams. What I'm saying is... I love you."

Fiona Frost is one of the recurring characters of the SPY x FAMILY series. She is a spy for WISE who acts under the codename "Nightfall". She works undercover as an office worker at Berlint General Hospital to support Loid Forger's cover story.

Fiona is a fairly tall and very attractive woman with a pale complexion and a slender figure. She has bobbed hair of a light lavender color and a long fringe covering much of the right side of her face, swept behind her left ear. Fiona has cold purple eyes framed by light-colored eyelashes and is most often seen wearing an emotionless expression. She is described as beautiful by Yor and has several admirers. Fiona's age is currently uncertain, but she appears to be younger than Loid.

Fiona is introduced wearing a dark blue pea coat with yellow (black in the manga) buttons, a purple sweater with turtleneck and black leather pants underneath, brown leather gloves, and ankle-high button boots with black heels when visiting the Forgers' apartment. Her stud earrings are black and take the shape of WISE's symbol.

When working in Berlint General Hospital as a psychiatric nurse, Fiona typically wears a long-sleeved dark blue sweater, a gray vest with black stripes and buttons over a white long-sleeved shirt, a black knee-length office skirt, dark gray tights, and black shoes with heels.

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 him. Fiona is deeply in love with Loid, to the point of being capable of compromising Operation Strix by visiting the Forgers' 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 Loid's superior partner and wishes to separate him from Yor so they can be together. However, her desire and love for Loid make Fiona unable to comprehend the bigger picture, as fulfilling them would cause suspicions and compromise Operation Strix.