As a Member of the Black Knights

The Black Knights are an organization that Lelouch (as Zero) created for the purpose of protecting the weak, freeing Japan from Britannian rule, and, eventually destroying the Britannian Empire as a whole. Kallen, Ougi and the rest of their rebel group became the first members of the Black Knights after being contacted by Zero. Zero proves his loyalty to them and provides them with new supplies, but many of the knights remain untrusting. They find it hard to believe that a masked man just wants to help them bring down Britannia. Kallen is key in convincing them to become the Black Knights. She trusts and believes in Zero because he saved her during their first battle.

The goal of the Black Knights is to defend the weak and they will even take on Japanese terrorists to reach this goal. Their first large scale mission is to save defenseless Britannian students and Princess Euphemia, who was hiding undercover among the hostages. This plan is key for Zero because it makes Cornelia unable to attack his group and it gives them recognition as an organization for justice, which in turn sets them apart from other rebel groups. The knights quickly gain the favor of the people and Kallen's faith in Zero grows. Zero makes use of Kallen's skills, giving her the newest and most advanced Knightmare the Guren Mk-II. He tells Kallen she's his "only ace". Kallen swears to follow Zero and begins to feel that he must have a good reason to hide his identity. She even goes so far as to put Zero's desire to stay hidden above her own desire to see him unmasked during the meeting with the Kyoto group.

The Black Knights are able to gain the support of the Kyoto group and Kallen continues to be a key member of the Black Knights and to be their ace pilot. Kallen is then given the title Captain of the Zero Squad, a position that makes her Zero's personal guard. Kallen continues to hold her place even when other skilled Knightmare pilots, including "Toudou the Miracle Worker" join the group. Kallen follows her orders to the letter, and even follows Zero to protect him during the Black Rebellion. Kallen fails to protect him because by the time she arrives Zero and Suzaku are facing guns towards each other. Suzaku shoots Zero's mask and Kallen finds herself in shock when she realizes that Zero was Lelouch the entire time. This gives Suzaku the chance to take Lelouch back to Britannia, while Kallen finds her beliefs shaken.

In Code Geass R2 Kallen is one of the few remaining Black Knights who weren't captured to be executed. She works with C.C. and the the others who had gotten away, they go undercover to restore Lelouch's memories, which were taken from him during his time in Britannia. She claims her reasons for saving a Lelouch, a man who she feels she can no longer trust after he hid the truth from her, is so she can confront him. Kallen claims that she'll only follow Zero, not Lelouch, and wants to think of them as two separate beings. She asks if she was controlled by Geass because she can't believe that she followed Lelouch but is told that her reasons were her own. The Black Knights are later exiled from Japan alongside Zero, from that point they are no longer a rebellion, but the official military of the "Nation of Japan".The purposes of the Black Knights change largely from there on out, and although their name is the same, they are no longer the original Knights. Kallen remains loyal to her name as a Knight until the end, fighting for what she believes in, and Japan which she swore to protect, Eventually going against Lelouch, and nearly dying for it.