Dolores Kesselbach, widow of the murdered millionaire Rudolf Kesselbach, belongs to the Malreich family, a lineage marked by hereditary mental instability. Her mother suffered from mental illness and her father was an alcoholic, contributing to her psychological deterioration. She exhibits homicidal madness driven by irresistible, unconscious impulses, manifesting in sudden violent acts where she eliminates perceived threats with precise cunning. Despite her madness, she experiences periods of lucidity showcasing strategic intelligence and acting ability.

Arriving at the Palace Hotel after her husband's murder, she performs the role of a grieving widow to avoid suspicion. Dolores employs disguises, including a fair wig and male attire as the shadowy figure "Louis de Malreich," enabling undetected movement. Her crimes encompass the stabbing deaths of Rudolf Kesselbach, a floor waiter, her associate Chapman, her brother Altenheim, her sister Isilda, and others like Gourel and Steinweg. She orchestrates these murders through meticulous planning, often using her maids Gertrude and Suzanne as accomplices or decoys.

Her primary objective involves securing Pierre Leduc, heir to the Duchy of Veldenz, intending marriage to reclaim her family's lost sovereignty. This ambition aligns with her husband's undisclosed schemes, though she only partially understands them. Her escalating conflict with Arsène Lupin becomes a prolonged struggle; she alternates feigning illness with actively pursuing him at night. She unmasks Lupin's identity as Prince Sernine, engineers his arrest, and expends substantial resources to thwart his efforts.

In a critical encounter, Dolores spares Lupin's life despite opportunity, revealing a complex behavioral shift. This restraint contrasts her relentless nature. However, Lupin later confronts her in a physical struggle, inadvertently causing her death. Her final moments highlight the tragedy of her condition, depicting her as both perpetrator and victim of uncontrollable impulses.