Laura Stuart is the former Archbishop of the Church of England and the head of its secret anti-magic organization, Necessarius, which is designated as the 0th Parish of the English Puritan Church. Despite holding one of the highest political and religious positions in the magical world, she appears as a young woman with an eighteen-year-old appearance, very long blonde hair that reaches well past her waist, and blue eyes. Little else is known about her true age or origin within the main story.

Her background is deeply intertwined with the history of the magical side. She is later revealed to be the body inhabited by the Great Demon Coronzon, an entity that successfully possessed the physical form of Aleister Crowley's second daughter, Laura, after Crowley's own attempt to take over that body was thwarted. She introduces herself publicly as Laura Stuart.

In terms of personality, Laura presents a deceptively cheerful and sometimes whimsical demeanor. She speaks Japanese in an unusual, almost comical manner because she learned it from Motoharu Tsuchimikado, a fact that other characters such as Aleister Crowley and Stiyl Magnus note as sounding strange. She has a fondness for baths and has collected the latest bathtub designs from Academy City, sent as gifts by Aleister to maintain diplomatic relations. Behind this lighthearted exterior, however, lies a cold, calculating, and highly manipulative mind. Stiyl Magnus describes her as someone who balances good and evil deeds so intricately that it becomes impossible to tell whether she is ultimately good or evil. She is said to be unmatched in manipulating human feelings, rationality, interests, ethics, and values, and is considered a hidden dragon among magicians who reached the top not through luck but through ruthless strategy.

Laura's motivations center on maintaining the power and influence of the Anglican Church, preserving the secrecy and control of magical knowledge, and furthering her own hidden agenda tied to the demon Coronzon. She is willing to sacrifice others, including her own allies, for what she perceives as the greater good or for her own ends. Her role in the story is that of a behind-the-scenes orchestrator who rarely involves herself directly in conflicts, preferring to let field agents like Kaori Kanzaki and Stiyl Magnus handle face-to-face encounters. She is responsible for ordering the memory-erasure protocol for Index and for designing the "John's Pen" mode that allows Index to use her grimoire library aggressively, all as a means of controlling Index's power.

Key relationships include her close friendship with Queen Elizard of England, the two having known each other for years. She works alongside Stiyl Magnus and Kanzaki, but her cold pragmatism often puts her at odds with them. Stiyl, in particular, is deeply wary of her and has expressed willingness to kill her if she threatens Index's life. Her relationship with Aleister Crowley is complex, involving both cooperation and underlying hostility due to their true identities.

Development is limited within the anime adaptation, as much of her deeper backstory and transformation are explored in later light novel volumes. In the events of A Certain Magical Index II, she appears as a supporting character who issues orders and provides information, but her true nature remains largely hidden. Her notable abilities are not directly combat-oriented; instead, her power lies in political manipulation, strategic planning, and knowledge of magic. She possesses a controller for Index's "John's Pen" mode and is able to override other signals. As the Archbishop, she commands the resources and authority of the entire Anglican Church and its anti-magic organization, making her one of the most politically influential figures in the series.