Kallen Stadtfeld, who also uses the Japanese name Kallen Kouzuki, is a central figure in the struggle against the Holy Britannian Empire. She is a young woman of mixed heritage, born as the daughter of a Britannian noble and a Japanese mother. This dual identity places her at the intersection of the conflict, allowing her to navigate both the privileged world of the conquerors and the oppressed reality of the Japanese people, known as Elevens. Outwardly, she is a student at the prestigious Ashford Academy, where she maintains a public persona as a frail and sickly young woman to disguise her true activities and avoid suspicion. This pretense is a source of personal frustration, as her natural personality is far more assertive and passionate.

Beneath this calm exterior lies a fierce and dedicated resistance fighter. Kallen is motivated by profound personal loss, primarily the death of her older brother, Naoto, who was a resistance leader. She is also driven by a desire to protect her mother, who chose to live as a servant in the Stadtfeld household to remain close to her daughter, a sacrifice Kallen initially fails to understand and harshly judges. This combination of grief, guilt, and love fuels her commitment to the Black Knights, the resistance organization led by the masked revolutionary Zero. Within the group, she serves as a member of Zero's personal guard and quickly distinguishes herself as its most talented and aggressive Knightmare Frame pilot.

Her loyalty to Zero is absolute, making her one of the most trusted and reliable members of the Black Knights. This devotion is severely tested when she discovers that Zero is, in fact, her seemingly indolent and dismissive classmate, Lelouch Lamperouge. The revelation creates intense internal conflict, forcing her to reconcile her disdain for Lelouch's public persona with her admiration for Zero's leadership and strategic genius. Ultimately, her loyalty to the cause and her growing personal feelings for Lelouch win out, and she continues to fight by his side, becoming one of the few people to learn the truth about his identity and Geass power.

The relationship with Lelouch is central to her character development. It evolves from simple respect for a commander to a complex mixture of admiration, trust, and unspoken romantic affection. She is often fiercely protective of him, yet is also one of the few people willing to challenge him or physically strike him when she believes he has lost his way. This dynamic puts her in a complicated position with other key figures, including a subtle rivalry with the immortal witch C.C., who shares a different, ancient bond with Lelouch.

Over the course of the second season, Kallen's skills are further sharpened as she pilots increasingly advanced versions of her signature red Knightmare, the Guren. Her abilities are widely acknowledged as being on par with the strongest pilots in the world, including the Britannian Knight of Round, Suzaku Kururugi. She fights in the climactic battles of the conflict, demonstrating not only her piloting prowess but also her strategic value as the Black Knights' ace.

In the series' final act, Kallen is forced to confront her feelings and her loyalty one last time. Even as she joins the global coalition to oppose Lelouch after he seizes the imperial throne, she is the first to grasp the true, sacrificial purpose of his plan, known as Zero Requiem. In the aftermath, she chooses to remember Lelouch not as a tyrant, but as the person she truly loved, and she works to build a peaceful future in the world he created. Her journey is one of reconciling conflicting identities, moving from a place of pure rage against an empire to a more nuanced understanding of sacrifice, justice, and personal conviction.