Fiona Frost is a supporting character in the anime and manga series 'Spy x Family.' She is a talented spy who works for WISE under the codename 'Nightfall.' Fiona is highly skilled, disciplined, and often maintains a cold, stoic demeanor. Secretly, she harbors strong feelings for her colleague Loid Forger (Twilight) and aspires to become his partner both professionally and personally. Her dedication to her missions and her complex emotions add depth to the series' espionage-driven plot.