As a Captain of a Zero Squad

Among the Black Knights, Kallen is labeled the Ace of the Black Knights or the Ace of the Elevens the title implies that she is the best Knightmare pilot in the entirety of the Black Knights, a title that, with a bit of trepidation, Kallen accepts, hoping that she can live up to the nicknames she was given. Being called an Eleven, even the Ace of the Elevens is a title that Kallen refuses to accept. Many of the Britannians that called her an Eleven received a thorough beating (i.e. Jeremiah at Narita).

Kallen is also known because of her official position in the Black Knights, she is the Captain of the Zero Squad, the leader of Zero's private forces who defend him and often lead the offense in important battles. She was told to go and help him in the Black Rebellion. While she attempts to defeat Suzaku and save Zero, even without the Guren, she ends up learning his true identity. The idea of which shakes her very foundations. Kallen remains the captain of the Zero squad and fights for that name. She even takes on Xing-ke when she knows the Guren may run out of power so that their plans can succeed. Kallen is captured for her attempts and Zero, knowing he puts his other forces in jeopardy sends them to retrieve Kallen because her skills in battles are unmatched.

Kallen even goes so far as to defend Lelouch when the other members of the Knights gain knowledge of his identity, showing her loyalty to Zero goes beyond the "betrayal" that the other Knights see, by him being a former Britannian Prince. She accepts that he can control people with Geass and realizes that their group needs him to succeed in saving the nation. She would have died defending him, if he hadn’t intentionally turned her against them. Kallen never betrays the Black Knights, and in theory she never betrayed her position as Captain of the Zero squad either. Kallen specifically stated that she wanted to think of Zero and Lelouch as separate beings. When Lelouch became the emperor of Britannia he turned his back on what Zero represented, and therefore, could no longer be considered Zero.