Kallen Stadtfeld is a red haired girl who lives a double life; one of a innocent schoolgirl, and the other, a terrorist. To most people, she is a frail girl who always misses school because of her sickness. However, she prefers to go by her Japanese origins, Kallen Kouzuki, who is a headstrong fighter and skilled Knightmare pilot, as well as a Britannian hater.
She is half Britannian and half Japanese, a fact that Lelouch discoverrs after Geass-ing her on school campus. Her biological mother, Mrs. Kouzuki is a Japanese maid who lives in Kallen's house. Her father, who is never seen in the series, is the owner of the household, and has a wife, who is Kallen's foster mother. The story is that her father had an affair with Mrs. Kouzuki and conceived Kallen as a result.
Kallen was very close to her brother, Naoto, who died in the beginning of the series. They were both terrorists working Ohgi's group. When Zero decided to have a meeting with their group, he becomes their leader, despite the fact that they do not see his face. Out of all the Black Knights, Kallen is the one who is most trusting of Zero, and remains one of his most loyal followers. She believes him because he saved her life in the Shinjuku Ghetto, and thinks that he is able to create miracles.
Living a double life is tough for Kallen, as she is tired of pretending to be frail and weak in school. She is very secretive about her identity, and will not hesitate to kill anyone who even suspects that she is a terrorist. She first encounters Lelouch on school campus, who used Geass on her to try to get some answers. After the Geass's effects worn off, Lelouch told her not to mention the Shinjuku Ghetto, which made her very suspicious.
She decides that she is going to kill Lelouch should he know anything, and meets him in the Student Council room. But she ends up being recruited into the council by Milly and the others.
Kallen is best known for doing a shower scene in episode 3, an event that sets her to be the fanservice girl for the entire series. When she was taking a shower in the dorm, Lelouch brought her clothes. She decides right then and there to get some answers from him, and asks him to bring her pouch into her hand. From behind the curtain, she grabs his hand and asks him about Shinjuku. Lelouch plays dumb the whole time, until they get a phone call from Zero, but it was actually a recording. After the phone call, she stops suspecting Lelouch of being Zero.
Kallen is chosen to be in the Zero Squad due to her skill and loyalty. She is more than happy to accept the job, and when she is given the Guren MK-II, she becomes even more dedicated to the cause of the Black Knights. With this new Knightmare, she was able to best any skilled pilot in the battlefield, including Jeremiah and Cornelia.