Henriette Mystere, alias Arsène, leads the Gentlemen Thieves faction within the Thieves' Empire. Her Toys ability allows her to cast illusions, utilized during heists and confrontations. She maintains a covert identity as the Student Council President at Holmes Detective Academy, hiding her role as Arsène.

Her primary motivation is a desire to battle formidable opponents, specifically Sherlock Shellingford of Milky Holmes. This compels her to orchestrate events aimed at restoring Milky Holmes' lost Toys, ensuring they regain full strength for challenging confrontations. Her commitment lies in creating worthy adversaries for battles of equal footing.

Physically, Henriette possesses a notably curvaceous figure, often accentuated by her attire outside of school uniforms. Her leadership of the Gentlemen Thieves is undisputed, and it is suggested her teammates harbor romantic feelings toward her. Her interactions with Milky Holmes blend antagonism with mentorship through manipulation, pushing them to regain their abilities.

Henriette consistently maintains her dual identity throughout the franchise, including the summer special. Her role remains constant as both a direct adversary to the detectives and a catalyst for their development. Her background and personal journey remain static across series installments. Her presence in the summer special aligns with her established persona, focusing on scheming to challenge Milky Holmes.

The character's origins, history prior to leading the Gentlemen Thieves, and deeper personal motivations beyond seeking thrilling battles are not elaborated upon. Her narrative function centers on creating conflict while paradoxically aiding the protagonists' growth.