Lucia serves as a Roman Catholic nun within the paramilitary Agnese Forces, led by Agnese Sanctis. She appears older than Agnese and Angelene, possessing short blond hair mostly hidden under a black coif, with locks visible at the sides of her face and vivid blue eyes. Her attire features a modified black scapular with a white collar and sleeves bearing a dark gray cross, detached at the lower section to reveal her legs, completed by black stockings held by a garter belt and brown loafers.

Her personality reflects extreme devotion to Roman Catholicism, often manifesting as intolerance towards non-Catholics, whom she labels heretics. This zeal drives her to unquestioningly follow orders, including deceptive collaborations like aiding the Anglican Church to capture Orsola Aquinas. As a sub-leader, Lucia displays ruthlessness, commanding nuns to pierce their own eardrums to counter Index Librorum Prohibitorum's "Sheol Fear" spell during the Battle of the Church of Orsola. Despite a cold exterior, she maintains deep loyalty to Agnese Sanctis and a complex, sisterly bond with Angelene, frequently criticizing Angelene's lax attitude toward religious duties.

Her background is largely undefined, though she is the oldest among Agnese, Angelene, and herself. She joined Agnese's force early, perceiving herself as divinely chosen for her role and exhibiting religious fervor from a young age.

During the Orsola Aquinas Rescue Arc, Lucia acts as an antagonist. She participates in the plot to capture Orsola under false pretenses and nearly kills Kamijou Touma after he inadvertently touches her shoulder, deeming it sacrilege. Her forces later besiege Touma's group at the Church of Orsola, where she devises the ear-piercing countermeasure against Sheol Fear. After the mission fails, the Roman Catholic Church scapegoats the Agnese Forces, leading to their imprisonment.

In the La Regina del Mare Adriatico Arc, Lucia and her comrades endure forced labor aboard the Queen's Fleet, confined in magic-suppressing robes. She and Angelene modify the robes to facilitate escape, though they are recaptured. Rescued by Touma and Orsola, Lucia initially distrusts them but relents after Orsola's persuasion. She later aids the Amakusa Christians in storming the flagship to save Agnese, slated for sacrifice in the "Rosary of the Appointed Time" spell. Lucia holds off former allies when Angelene is injured. Post-conflict, she defects to the Anglican Church's Necessarius alongside the Agnese Forces.

After joining Necessarius, Lucia retains her Roman Catholic faith but severs ties with the Vatican. She represents Roman Catholicism in England under Agnese's leadership. Her demeanor softens; while remaining devout and occasionally critical of Angelene, she shows greater tolerance toward non-Catholics. She participates in activities like laundering clothes with Orsola, playing poker with comrades, and expressing curiosity about other cultures, leading to misunderstandings. During the Anglican Church–Russian Orthodox Church conference, she engages in banter with Angelene about physical appearance and samples Japanese umeboshi as a cultural novelty.

In the British Royal Family Arc, Lucia assists Agnese Forces in investigating the New Light group in Edinburgh. Surrounded by 400 knights during the English Civil War, she notes the absence of civilian evacuation protocols, indicating a large-scale knight invasion. Agnese then activates a spell using her Lotus Wand, implicating Lucia in the confrontation.