Arsène, also known under the civilian identity Henriette Mystère, is the leader of the Gentlemen Thieves, often called the Thieves’ Empire. By day, she poses as the Student Council President at Holmes Detective Academy, using this position to monitor the school. Her primary power involves “Toys,” supernatural abilities that grant her complex illusions, which can later evolve into tangible entities under her control. Possessing significant strength that allows her to commit undetected thefts, she deliberately restrains herself during crimes, driven by a desire for worthy opponents. She specifically seeks battles against the detective group Milky Holmes at their full potential, creating scenarios to pressure them into regaining their lost abilities for challenging confrontations. Throughout the series, she orchestrates events such as relocating the detectives to a dilapidated attic, setting fire to their residence, and staging her own kidnapping as Henriette to provoke a desperate rescue. After a climactic battle where her true identity is revealed, she retreats, postpones the detectives’ expulsion, and refurbishes their quarters. Her commitment to facing a fully powered Milky Holmes remains consistent, as she consciously limits her actions despite being capable of stealing any treasure. As Henriette, she wears standard school attire, while her Arsène persona features flamboyant outfits often designed to emphasize her physique. She commands the loyalty of her subordinates—including Stone River, Twenty, and Rat—who infiltrate the academy under various covers such as janitor, chef, teacher, and student. Her long blue hair and violet eyes contrast with capes, thigh-high boots, and regal accessories like crowns or masks, blending academy refinement with thief opulence.