Age: 17 (season 1) 18 (season 2)
Zodiac Sign: Aries
Blood Type: B
Height: 173 cm (5'8")
Birth date: March 29, 2000

A Britannian-Eleven girl, who believes herself to be a Japanese person at heart. Before the Britannia Empire conquered Area 11, Kallen's original name was Karen Kouzuki. She comes from a prestigious family and attends the Ashford Academy where she receives outstanding grades while serving as a member of its Student Council. When away from the Academy, she fights as an anti-Britannian operative. Also, Kallen helped organize the Black Knights.

Her brother, Naoto Kouzuki, with whom she had a deep connection, was a former leader of her resistance cell 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 arrested for using Refrain (a fictional 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.

Frail and meek, this aspect of Kallen's character was used as a public front to mask her true nature as well as hide her involvement in the resistance.

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 school, she portrays herself as a weak-bodied high school student who obeys all of the formalities, and goes by her Britannian name of Stadtfeld. However, her personality changes in to a very strong-willed and forceful one when she acts as a resistance fighter.

As a running gag, Kallen ends up nude 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, she is forced to dress in an immodest bunny costume at the start of the second season (to plant a tracking device on Lelouch), and she walks in on a meeting between C.C. and the Chinese ambassadors wearing only a towel when intending to complain about the previous situation to C.C. (claiming that the operation would've gone faster if C.C. was wearing the outfit).

In an interview reported by the anime magazine Newtype USA, seiyu Ami Koshimizu talked about the difficulty of voicing Kallen because of the double life that her character leads. 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.