Kallen Kozuki was born before the Second Atlantic War, in which Japan fell to Britannia and was renamed Area 11. Her father was a Britannian nobleman of the Stradtfield family and her mother was a simple unskilled workers. However after the war, any relations with Elevens became taboo and Kallen and her brother Naoto went to live to live with her mother. However when Mr. Stradtfield's new wife was revealed to be infertile, Kallen's mother pushed for Kallen to be the heir to the Stradtfield family, and stayed as a maid so that she could look after her daughter, although Kallen believed it was just because she couldn't let go of a man she loved a long time ago. Her brother Naoto meanwhile became the leader of a small terrorist group which fought against Britannia for independence.

However when her brother Naoto died, his friend Kanami Ohgi became the leader of the group and Kallen joined in order to honour her brother and his dream. Eventually the terrorist cell captured Code-R's test subject C.C. under the disguise of poison gas. Eventually C.C. comes into contact with Lelouch and gives him the power of Geass which he uses to escape from the military and lead Kallen and Ohgi's group to victory over the much larger Britannian forces. From there Ohgi's group becomes the Black Knight's headed by Lelouch under the mask of Zero.

The Black Knights then go on to become Britannia's greatest threat with Kallen in the inner circle, and as the leader of Zero's personal forces. In the First Battle of Tokyo she is unable to save the unmasked Lelouch from Suzaku. She then leads the attempt to rescue Lelouch from the OSI at Babel Tower as the only one who knows who Zero truly is. However as events unravel what will her final role be?