Kallen Stadtfeld, who prefers to be known by her Japanese name Kallen Kozuki, is a central character from the second season of Code Geass. She is a young woman of mixed heritage, born to a Britannian father from the prestigious Stadtfeld family and a Japanese mother. This dual identity places her at the heart of the series' central conflict, living as a member of the occupying Britannian nobility while secretly fighting for Japan's liberation as part of the resistance group that would become the Black Knights.

On the surface, Kallen leads a double life as a student at Ashford Academy. There, she adopts the persona of a frail, sickly, and reserved young woman to mask her true activities and avoid suspicion. This facade is a source of personal frustration, as her real personality is vastly different. In truth, Kallen is strong-willed, determined, and possesses a fierce, sometimes impulsive and short-tempered, spirit. She is deeply courageous and driven by a powerful sense of justice, fighting for what she believes is right with unwavering conviction. Beneath her tough exterior lies a compassionate and sensitive person, particularly toward her loved ones, a trait that reveals itself more as she grows throughout the story.

Her primary motivation stems from personal tragedy and a desire for a better world. She is initially driven to join the resistance to honor the memory of her older brother, Naoto Kozuki, a former resistance leader who was killed by Britannian forces. Her resolve deepens significantly after learning the truth about her mother. Her Japanese mother, who is employed as a maid in the Stadtfeld household, endures mistreatment to remain close to her daughter. Initially, Kallen feels shame and frustration toward her mother for this, but upon discovering her mother's sacrifice, she finds a new purpose: to create a world where they can live together with dignity, free from oppression.

Within the Black Knights, Kallen serves as the captain of the Zero Guard, the personal guard of the revolutionary leader Zero. She is celebrated as the organization's undisputed ace pilot in the Knightmare Frame mecha, a position that makes her an indispensable asset in combat operations. Her loyalty to Zero is absolute, ranking among the highest in the organization, and she becomes one of the few people to learn his true identity as Lelouch Lamperouge, her classmate at Ashford Academy. This revelation creates a complex dynamic; she confronts Lelouch, fearing his Geass might have manipulated her loyalty. After he insists her dedication is genuine, she continues to support him, though her feelings evolve from mere admiration to a deep, personal affection. Her care for him is shown when she intervenes during a moment of despair, refusing to let him escape his responsibilities and pushing him back onto his path. In the final stages of the R2 season, she feels betrayed by what she sees as Lelouch's despotism, only to be one of the first to understand the sacrificial nature of his ultimate plan, the Zero Requiem, leaving her to mourn him while cherishing the peaceful world he created.

Her most notable ability is her exceptional skill as a Knightmare Frame pilot. She is widely regarded as one of the most talented pilots in the series, possessing a natural aptitude and tactical acumen that allows her to match even the elite Britannian Knight of Round, Suzaku Kururugi. Her technical prowess is showcased as she pilots successive versions of the Guren, a powerful and highly advanced series of Knightmares. Starting with the Red Lotus Guren Type-02, which features a devastating "Radiation Wave" arm cannon, she later pilots the flight-capable Guren Flight-Enabled Version and finally the ultimate Guren S.E.I.T.E.N. Eight Elements, which she uses to achieve victory against Suzaku in the series' final battle. Her combat effectiveness remains at maximum regardless of the machine she operates, cementing her status as a legendary figure in the Black Knights' revolutionary war.