Keith Harcourt, known to the public as the Phantom Thief Black Rose, is a seventeen-year-old aristocrat and the older twin brother of Francis Harcourt. Living a double life, he conceals his true identity behind a mask and a black cape, operating as a notorious thief who targets the wealthy and privileged classes. His actions are driven by a deep-seated belief that the upper class is largely frivolous and self-centered, and he redistributes the stolen wealth to those living in poverty. This crusade positions him as a kind of Robin Hood figure, celebrated by the common people and a source of fascination for journalists, while he himself carries the burden of being an aristocrat who despises his own station.

Keith's personality is markedly different from his brother's open and warm demeanor. He is characterized as cold, quiet, and emotionally withdrawn, having become a loner who operates outside the bounds of his noble family. This cynical and disenchanted outlook stems from his childhood. While the more innocent Francis remembered only the happier, sunnier side of their late mother, Keith witnessed her secret unhappiness as a bird in a gilded cage, trapped in a marriage that stifled her desire for freedom. This perception fuels his rejection of traditional noblesse oblige, which he views as an insufficient remedy for systemic inequality, and drives him to take more direct, albeit illegal, action.

The central irony of Keith's life is his role as the Starry-Eyed Knight. It was he, not Francis, who rescued the protagonist Nadja from a fire at the Applefield Orphanage, an act that became a cherished, defining memory for her. He recognizes Nadja's carefree and open spirit as a reflection of the freedom his mother longed for, and he becomes her self-appointed protector. He warns her to guard a precious brooch that holds the key to her lineage, and he endures significant hardship for her sake, including being captured and tortured by an antagonist named Hermann, suffering lashings in an attempt to retrieve her keepsake.

The existence of Keith creates a profound emotional conflict for Nadja, as she struggles to reconcile her feelings for the two identical twins. She initially confuses Keith for Francis when they meet, and the revelation that her heroic Starry-Eyed Knight and the charming aristocrat she has been traveling with are two different people throws her into a state of confusion about her own heart. Both brothers fall in love with Nadja, which creates a silent and painful rivalry. Keith often believes his love is unrequited, as Nadja’s interactions with Francis lead her to believe she is in love with her knight, not knowing the identities were swapped. His suffering is compounded by his discovery of a secret emblem on Nadja's ring, revealing her true identity as a Preminger, one of the richest heiresses in Austria. Knowing the pain his own mother endured, he desperately wants to prevent Nadja from being trapped by a similar aristocratic fate.

As the true Black Rose and the real Starry-Eyed Knight, Keith possesses considerable skill as a thief. His trademarks include sending a calling card to taunt his victims before a heist and leaving a single rose at the scene of his crimes. His abilities extend to infiltration, acrobatics, and escape, allowing him to move through high-society events and mansions undetected. Throughout the story, he remains a steadfast, if tormented, ally to Nadja, ultimately valuing her freedom and happiness above his own desires.