Kallen Kozuki is the tetartagonist of the anime series Code Geass: Lelouch of the Rebellion. She is officially known as Kallen Stadtfeld, and goes by this name at school, but prefers her mother's maiden surname during her revolutionary activities.
Kallen is a young Britannian-Japanese woman, who believes herself to be a Japanese person at heart and prefers to be known by her Japanese mother's name. Her father is Britannian and her mother is an Eleven; her father comes from a prestigious Britannian family, which Kallen benefits from. She attends the Ashford Academy, where she is a member of its student council and is one of its most outstanding students. Her brother, Naoto Kōzuki (紅月ナオト, Kōzuki Naoto), with whom she had a deep connection, was a former leader of the resistance organization before his death. Both Kallen's stepmother, a Britannian, and biological mother live with her in the same house, but her biological mother is employed as a maid and is often mistreated by both her stepmother and Kallen herself. After her biological mother is injured and later imprisoned for falling prey to the Refrain drug (which lets people think they're reliving the past), Kallen changes her views and decides to fight against the Britannian Empire not only for her brother, but to fulfill her new wish for a world in which she can live happily with her biological mother. This aspect of her character contrasts the frail-girl image she uses as a public front. Kallen leads a double life as a normal Britannian student and as a member of a guerrilla resistance group against Britannia, which is later reformed into the Order of the Black Knights by Lelouch Lamperouge. In episode 3 it is revealed that she skips school regularly as a result of participating in terrorist activities and with an excuse of poor health. When she is in school, Kallen portrays herself as a weak-bodied high school student who obeys all of the formalities. However, her personality changes in to a very strong-willed and forceful one when she acts as a resistance fighter. When she feels under suspicion she often readies the knife that she conceals in a small pink pouch, showing what she will do to keep her identity secret. As a running gag, Kallen ends up naked or immodestly dressed in front of other people on numerous occasions. Lelouch sees her naked after she pulls back a shower curtain in surprise in the third episode, Suzaku sees her bathing under a waterfall when they are stranded on Kaminejima Island, she is forced to dress as a Playboy Bunny at the start of the second season, and she walks in on a meeting between C.C. and the Chinese ambassadors wearing only a towel when attempting to complain about the previous situation to C.C. Another possible running gag is her relationship with Lelouch as they are set up in scenes where they almost kiss but always get interrupted. In an interview reported by the anime magazine Newtype USA, Seiyuu Ami Koshimizu talked about the difficulty of voicing Kallen because of the double life that her character had. Certain aspects of Kallen's character such as her girlish naivete and competitiveness made it easy for the voice actress to relate, even to the point of admitting that in some ways they act very much the same.
Kallen appears early in the first episode, driving a truck with C.C.'s capsule, and attempts to fight off Jeremiah's Purist Faction using a Glasgow. After Lelouch gains possession of his Geass from C.C., he takes possession of an enemy Sutherland and helps Kallen to escape the other Knightmare Frames chasing her. He further directs the remaining resistance forces, granting them a decisive advantage until Suzaku Kururugi uses his Lancelot to disable their Knightmares. She sacrifices her own Knightmare in return for helping her earlier against Jeremiah to allow Lelouch to escape, despite not knowing who he really is. She is able to escape after ejecting, meeting up with Ohgi and the rest of the resistance. However, the Britannian Military found where they were hiding. When the commander was just about to issue the order to fire, the Viceroy, Clovis la Britannia orders a ceasefire saving Kallen and the others.
Lelouch uses Geass on Kallen. After this incident, Kallen returns back to school for the time being. She returns back to class with a fake persona of a frail girl. After Lelouch discovers that Kallen, one of the terrorists he helped in Shinjuku Ghetto, attends the same school, he attempts to confirm his suspicions. Lelouch's unfamiliarity with his Geass however, causes him to reveal knowledge of the Shinjiku incident to Kallen, leading her to suspect him of being their unknown benefactor. As they both plan on exposing the other's suspicions, the Student Council throws a party for Kallen. During their explanation as to why, allows Rivalz to propose a toast. Shirley stating that they are too young for it, causes Rivalz to throw the bottle to Lelouch. With Lelouch having the bottle, Shirley attempts to retrieve it from him. However, the cork pops open spraying all over Kallen. Kallen then takes a shower and uses this situation to her advantage in order to find out if Lelouch really is Zero. Lelouch creates a cunning deception to ease her suspicions of his identity by playing a recorded message of himself on the telephone with the help of a Geassed Sayoko, drawing her suspicions off him. When the death of Prince Clovis is announced in the media, it is Suzaku who is accused of the murder. After receiving the message from Zero, both Kallen and Ohgi decide to help him save Suzaku Kururugi who was falsely accused of murdering Clovis. Both have their doubts as the operation is going, but Zero uses his own strategy and his Geass to turn the situation in his favor resulting in them to believe in Zero.
Arthur, a stray cat, is later chased throughout the school grounds by the entire student body after wandering into the Lamperouge residence and escaping with Lelouch's Zero mask. Milly announced that the one who would catch the cat would get a kiss from any member of the Student Council. Suzaku and Lelouch were the ones that got the cat and got a kiss on the cheek from Nunnally. The Student Council adopts him and builds a cat house in the council room. Following this incident, Kallen along with the rest of the student body watches Prince Clovis' funeral on live. During the funeral, the Emperor of Britannia, makes a speech on the virtues of inequality and the social battle for evolution and betterment.
Following Lelouch's failed attempt at stopping the new viceroy of Area 11, Cornelia li Britannia, the Japan Liberation Front decide to act and in turn kidnapped Britannians in a hotel jacking. Unable to stand by, Zero decides that this event can be the starting point for a new organization. After saving the hostages, Zero uses the media in order to publicly announce the formation of the Black Knights with the core members present.
Kallen's Family Estate
With the formation of the Black Knights and Zero's increasing number of victories over the Britannian forces, Kallen comes to greatly admire him. Later on, Kallen is seen back at her house and for the first time is shown her step-mother, Mrs. Stadtfeld and as well as Mrs. Kōzuki who has become a maid in order to stay with her daughter. During one of the Black Knights operations to stop the distribution of the drug, Refrain (Drug), Kallen notices her mother as one of the people using it. She attempts to save her using her Glasgow, but a corrupted member of the Britannian police force uses a Knightpolice in order to kill Kallen. Kallen then ensues in a battle with it and eventually defeats him. Afterwards, Kallen realizes the truth surrounding why her mother decided to stay with her all this time and promises her that she will take part in making a world where they can both live in. The Guren Mk II After losing her Glasgow, Zero gives Kallen the newly-developed Guren Mk-II to replace it. As Zero decides to attack Cornelia in the Narita Mountains, all members of the Black Knights head to Narita. Kallen starts off the operation using the Guren's Radiant Wave Surger to start a landslide that will decimate the Britannian forces. Afterwards, Kallen effectively starts to destroy the remaining forces including most of the Purist Faction. She easily outmatched Jeremiah and dispatched Kewell with ease as well. She then moves on to her next checkpoint which is to trap Cornelia. She initially has the advantage and overwhelms Cornelia until Zero arrives to talk with Cornelia. As they prepare to capture her, Suzaku appears with the Lancelot in an attempt to save the Viceroy. Zero enraged that the Lancelot is again interfering with his plans orders Kallen to destroy. Kallen engages in a one on one fight with Suzaku having an advantage with the Guren. As they continue fighting, Suzaku is shocked at the power of the Guren, but is unwilling to give up. He fires a shot from his VARIS which in turn Kallen attempts to block with the Radiant Wave Surger. However, the ground beneath her gives away, causing her to fall and damage her Knightmare. Because of this, Zero orders all Black Knights to retreat leaving the JLF to fend for themselves giving much discomfort to Kallen, Ohgi and Tamaki. Before the Black Knights begin their operation to rescue Tohdoh from his scheduled execution, Rakshata appears to meet Zero and give them the new Knightmare models and suits. Kallen goes along with Zero to retrieve Tohdoh. After reuniting with the Four Holy Swords, Suzaku appears once again with the Lancelot in order to stop them from leaving. During their fight, reinforcements appear causing Tohdoh to issue the command to release the chaff-smoke which Kallen does so and retreats. Now that the Black Knights are growing, Zero announces the chain of command and gives Kallen the position of being captain of the Zero Squad which is Zero's personal force. Hearing that Suzaku is appointed Euphemia's knight, the Black Knights decide on what his fate shall be because of his rising popularity with the Elevens. Diethard decides to take his own course and hints to Kallen to assassinate Suzaku in order to please Zero. She attempts to assassinate him during his party, but is interrupted by Lelouch.
Afterwards, she is seen during the battle at Shikine Island in order to capture Suzaku. The plan goes wrong when Schneizel appears with the Avalon and fires at Zero and Suzaku with the Hadron Cannon. Later, Kallen appears to be alive and is found by Suzaku who arrests her for being a member of the Black Knights. The following day, Kallen and Suzaku run into Zero and Euphemia. The floor below them collapses giving Zero and Kallen a chance to escape with the Gawain. Kallen shocked to see that Lelouch is the true identity of Zero. At the end of the first season, Kallen reveals her identity to the students of Ashford Academy when the Black Knights occupy it as their base of operations, convincing the student council that they will not be harmed. When Zero abandons the battle to rescue Nunnally, Kallen follows him. She witnesses his confrontation with Suzaku and learns about his true identity, the truth about the massacre of SAZ, and his Geass power. She was also shocked to hear that Lelouch admitting that he used the Japanese, including her, for his scheme. She flees in shock rather than aiding Lelouch to deal with Suzaku.
- Ami Koshimizu won "Best Actress in supporting role" for her role as Kallen at the first Seiyu Awards in 2007.
- Kallen was awarded 8th place in the 29th Anime Grand Prix, then 17th and 16th, respectively, in the following two.