Lucius Oriflamme, known as Luca, is a young vampire of high political standing, heir apparent to the title of Grand Duke within the noble House Oriflamme. His uncle, Lord August Ruthven, manages most duties, leaving Luca with limited responsibilities. Luca appears as a barely adolescent boy with short red-brown hair featuring two cowlicks and thin bangs. His round face, large upturned green eyes, and long eyelashes enhance his childlike look. He often wears formal attire, including a green, silver-detailed mantle resembling a houppelande, black shorts, a white shirt, and a hat.

Personality-wise, Luca blends maturity and youthful vulnerability. He strives to appear calm, responsible, and dignified to match his status, suppressing childish tendencies. Yet, he exhibits intense emotional reactions when loved ones are threatened, especially Jeanne. He clutches his clothes, bag, or hands when distressed, angry, or flustered. Naïveté surfaces in social interactions, and he can be shy around those he admires.

Luca's core motivation is protecting loved ones. His older brother, Loki Oriflamme, a curse-bearer, is imprisoned. To cure Loki, Luca seeks the Book of Vanitas, leading him to confront Vanitas and Noé Archiviste in Paris. When Vanitas refuses the book and insults Luca, he orders Jeanne to retrieve it by force. After Jeanne is subdued, Luca reacts violently upon witnessing Vanitas kiss her without consent, unleashing his pyrokinesis to escape Noé's restraint.

Later, at the Bal Masqué, Luca apologizes to Noé for his earlier aggression, acknowledging his emotional immaturity. He witnesses Charlatan's attack and watches helplessly as Noé is tormented by Naenia, expressing relief when Lord Ruthven intervenes.

Luca regularly visits Loki and attends senate meetings discussing vampire affairs. During deliberations about the Gévaudan incident, he breaks his silence to vehemently oppose executing Jeanne, asserting his authority as her master to shield her. After Jeanne returns, Luca listens to her account, becoming furious upon learning she spent a night alone with Vanitas. He grows alarmed when Jeanne describes her growing romantic feelings for Vanitas, leading him to seek Dominique de Sade's assistance.

Luca shares close bonds: he idolizes Loki, describing him as his "precious person" and relying on him for emotional security, instinctively clutching Loki's clothing when frightened. His relationship with Ruthven is respectful but distant; Luca avoids overt displays of affection from his uncle. With Jeanne, his chevalier and former bourreau, he exhibits fierce protectiveness and dependency, prioritizing her safety.

His abilities include typical vampire enhanced speed, strength, and endurance, plus pyrokinesis, generating explosive flames from his body during extreme emotional distress, though he lacks full control.

Luca demonstrates subtle growth, reporting delightedly that he has grown slightly taller and undergoing measurements for an upcoming ceremony. He continues balancing noble obligations with personal loyalties, commissioning Jeanne to investigate disturbances in Paris.