C.C. is a central figure in the story, introduced as a mysterious, green-haired girl held captive as a test subject within a Britannian military research facility. Her true name is never revealed to the audience, with C.C.—pronounced "C-Two"—serving as her only known identifier. She possesses the ancient power of the Code, which grants her immortality and the ability to bestow the supernatural power of Geass upon others. It is through a contract with the protagonist, Lelouch Lamperouge, that she gives him the Geass of "Absolute Obedience," setting the series' central conflict in motion.

Her personality is shaped by centuries of existence. She is often sardonic, stubborn, and appears emotionally detached, viewing the world through a lens of nihilism born from endless, lonely years. Despite her aloof exterior, she displays a dry, cold wit and a willful, unapologetic nature, frequently doing as she pleases without regard for the inconvenience it causes others, particularly Lelouch. This behavior masks a profound sense of isolation and a deep-seated weariness with her own immortality. Her primary motivation, revealed over the course of the narrative, is not power or conquest, but a desire to end her own existence. Having lived for centuries and witnessed countless deaths, her greatest wish is to be released from the curse of eternal life.

Within the story, C.C. acts as Lelouch's protector, confidante, and strategist. She serves as his accomplice, often acting as a decoy by wearing his Zero costume or assuming command of the Black Knights in his absence. Her role is that of an enabler and a witness, having set Lelouch on his path and ensuring he survives to fulfill their contract. Following the Black Rebellion, where her Knightmare Frame, the Gawain, sank into the ocean, she is secretly rescued and goes into hiding with Kallen Stadtfeld, waiting for the opportunity to reconnect with Lelouch.

Her most significant relationship is with Lelouch, her contract partner. While she initially views him as a tool to achieve her own death, their bond deepens into a complex partnership built on mutual dependence. She is one of the few people who knows his true identity and stands by him until the end, never betraying him. Her past is also tied to Lelouch's mother, Marianne, with whom she had a previous Geass contract, and the rogue Geass user, Mao, whose obsession with her leads to tragedy. She also has a contentious but eventually cooperative relationship with Suzaku Kururugi, who comes to call her "Lelouch's shield".

C.C. demonstrates significant development from a passive, cynical immortal seeking only death to an individual who finds a sense of purpose and connection. When her memories are lost following an encounter with the Emperor, she regresses to a frightened slave girl, highlighting how her immortal existence has fundamentally changed her. Her development culminates in her acceptance of her role as a partner and witness to Lelouch's plans, finding a form of solace in their shared journey.

Her notable abilities are extensive. As a Code bearer, she is functionally immortal, immune to aging and conventional death, capable of healing from any wound, including gunshots and other fatal injuries. She is also immune to the effects of Geass. She can grant Geass to others and can sense the presence of those who possess it. Despite her supernatural gifts, she is also a competent leader, a skilled tactician, a capable hand-to-hand combatant, and a proficient pilot of Knightmare Frames. In her more mundane moments, she is known for her obsessive fondness for pizza, which she goes to great lengths to acquire, a quirk that provides occasional levity.