Fiona Frost

thumb|left|Fiona internally proclaims her love for Loid Fiona presents a consistently stoic, controlled demeanor, a habit reinforced by Twilight's instruction to always suppress her emotions. Internally, however, she experiences strong feelings, most notably her romantic devotion to Twilight. She regards him as the same person regardless of the identity he assumes and believes that becoming his wife would form the ideal partnership, both emotionally and operationally. Fiona views herself as the most suitable, if not the only, candidate for that role. These feelings are most apparent in her attitude toward his Operation Strix persona, Loid Forger. Her attachment often leads her to consider replacing Yor as Loid's wife, a possibility she has raised with both Loid and their handler, Sylvia Sherwood. Although Fiona sees this outcome as advantageous for Twilight and the mission, she is aware that acting on her desires would create suspicion and jeopardize Operation Strix. This tension creates an ongoing conflict between her personal devotion and her duty as an agent, as her loyalty to Twilight can cloud her judgment and interfere with her ability to fully prioritize the broader objective. thumb|Fiona's cold demeanor isolates her from most of her WISE coworkers Among most of her WISE colleagues, Fiona has a reputation for ruthlessness and excessive ambition, and she is often perceived as someone willing to advance at others' expense. While she can employ morally questionable tactics, her drive is motivated less by personal gain than by a desire to prove her worth to Twilight. Her perfectionism manifests in strict self-discipline and a readiness to endure physical and emotional strain, which she treats as necessary sacrifices rather than warning signs. thumb|left|Fiona's rage after Wheeler threatens to kill Twilight Fiona typically keeps her emotions tightly contained, but they surface under extreme circumstances. During the Campbelldon tennis tournament, she responds with unusual aggression after Loid is mocked by the Bolic Brothers, retaliating with excessive force. Later, when she challenges Yor to a tennis match in an effort to demonstrate her superiority, she is visibly shaken by Yor's unexpected strength and deeply affected by her defeat. Afterward, she breaks down in private before doubling down on her determination rather than reconsidering her approach. Similarly, when Twilight's life is threatened by Winston Wheeler, Fiona abandons her usual restraintand openly expresses her feelings as she defeats him in a fit of anger. She is also prone to embarrassment when her feelings are exposed, such as when she realizes Sylvia is aware of her attachment to Twilight. thumb|Fiona visits Franky for a new commission During missions involving Twilight, Fiona often pushes herself beyond safe limits in an effort to perform flawlessly. In the Campbelldon operation, she continues competing despite injuring her hands, prioritizing results and Twilight's approval over her own well-being. She is highly distrustful and willing to manipulate others when necessary, such as coercing a counterfeiter into assisting her by orchestrating an SSS ambush after tipping them off. Despite these tendencies, Fiona remains a highly capable operative who can work effectively with others, gradually developing a functional level of trust with fellow WISE agents and allies such as Franky Franklin.

Fiona is a fairly tall and very attractive woman with a pale complexion and a slender figure. She has bobbed hair of a white color (lavender in manga) and a long fringe covering much of the right side of her face, swept behind her left ear. Fiona has dark gray eyes (purple in manga) 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.