Keith Harcourt is a seventeen-year-old aristocrat who leads a double life as the infamous phantom thief known as the Black Rose, or Rosa Negra. He is the older twin brother of Francis Harcourt, though the two are identical in appearance. Unlike his brother, who believes in using his noble status to help the less fortunate through charity, Keith has chosen a far more radical and cynical path to achieve justice.

Keith’s personality is markedly cold, reserved, and quiet, a stark contrast to the more open and gentle nature of his twin. This demeanor stems from a deep-seated disillusionment with the aristocracy. While Francis only saw their mother’s kind and happy side, Keith witnessed her secret unhappiness and felt she was trapped as a bird in a gilded cage, unable to travel freely. Her early death and the arguments between their parents had a profound impact on him, leading him to believe that the wealthy upper class is largely frivolous and self-centered. His core motivation is to correct this imbalance, not through noblesse oblige, which he views as insufficient, but by directly stealing from the greedy rich and distributing the wealth to the poor, much like a Robin Hood figure.

In the story, Keith operates from the shadows, using the persona of the Black Rose to carry out his deeds. His trademarks include a black costume with a cape and mask, a calling card sent to taunt his victims before a heist, and leaving a black rose at the scene of his crime. He is a mysterious figure, beloved by the common people and a subject of fascination for journalists. His path crosses repeatedly with the protagonist, Nadja, as she travels with the Dandelion Troupe. Initially, she disapproves of his methods, but over time she comes to understand his motives and feel empathy for him.

The most pivotal revelation regarding his role is that Keith is the true identity of Nadja's "Starry-Eyed Knight," the person who rescued her from a fire at the Applefield Orphanage in her childhood. This creates a significant emotional conflict for Nadja, who had long believed her mysterious rescuer was Francis, with whom she had also shared many pleasant moments. The existence of two identical Harcourt twins, one of whom is her romantic ideal and the other a constant companion, throws her feelings into confusion.

Key relationships define his character arc. His bond with his brother Francis is complex; they share the same ultimate goal of equality, but their methods differ drastically. Keith is deeply envious of the more innocent and honorable path Francis can walk, and at one point Francis even impersonates the Black Rose to protect Keith’s secret. Most significantly, Keith falls in love with Nadja, primarily because her carefree spirit and fierce love of freedom resonate with his own desires. He appoints himself her secret protector, constantly watching over her and intervening when she is in danger. However, his love is often portrayed as a source of suffering, as he believes his feelings may not be reciprocated due to Nadja’s apparent affection for Francis.

Throughout the series, Keith undergoes subtle but important development. While he remains set in his belief that stealing is necessary, his interactions with Nadja soften his cynical edges. His dedication to her is absolute, leading him to endure severe torture, including being whipped while imprisoned, in order to retrieve her precious brooch. He also discovers the secret of Nadja’s heritage, learning that she is actually a member of the wealthy Preminger family, and warns her to guard the proof of her identity carefully. Despite his cold exterior, his actions consistently reveal a deeply romantic and self-sacrificing nature, willing to endure pain and remain in the shadows to ensure the happiness and safety of the girl he loves.