C.C. is a central figure in the narrative, an immortal being whose existence is defined by the burdens of her unique powers and a past shrouded in centuries of isolation. Her real name is never revealed, and she is known only by the pseudonym C.C. Her exact age is unknown, but she confirms having lived for hundreds of years, having witnessed historical events such as the American Revolution and having formed contracts with countless individuals across different eras. Her long, light green hair and golden eyes, combined with her often detached and sardonic demeanor, give her an otherworldly and enigmatic presence.

The foundation of C.C.s character is her immortality, granted to her by a Code. Before obtaining this power, she was a young, orphaned slave who was given the Geass ability to be loved by everyone. This power, however, brought her no happiness, only hollow devotion from others. The nun who gave her this Geass eventually tricked and coerced a young C.C. into taking her Code, an act that transferred the nun's immortality and desire for death onto C.C. while robbing her of her own Geass. This traumatic event left C.C. cursed to live through the ages, unable to die by any conventional means, having survived burnings, beheadings, and other executions throughout history. This existence has made her deeply cynical, lonely, and nihilistic. She often appears indifferent to the world around her, finding solace only in simple, modern pleasures like pizza, for which she has a well-known fondness.

In the main storyline, C.C. is initially a captive of the Britannian military before encountering Lelouch Lamperouge. When he is caught in a deadly skirmish, she makes a contract with him, granting him the power of Geass in exchange for a single condition: he must fulfill her greatest wish, which is to end her immortal life. Her role in the story quickly evolves from a mysterious benefactor to Lelouch's closest and most vital accomplice. She serves as his confidant, strategist, and protector, often piloting the Knightmare Frame Gawain alongside him and even impersonating his Zero persona to protect his identity. Within the Black Knights, her unofficial but powerful position as Zero's right hand creates an air of mystery and occasional friction with other members, who are unaware of her true nature.

C.C.s central motivation is her desire to die. Having grown weary of her endless life, she sees her contract with Lelouch as a potential path to this goal. However, her relationship with him fundamentally alters her perspective. Unlike her previous contracts, Lelouch does not fear or revile her for what she is. When she confesses her loneliness, he famously declares that he is her accomplice, and that if she is a witch, then he shall be a demon. This shared bond of being outsiders, both wielding power that isolates them, becomes the core of their partnership. Her development throughout the series is a gradual shift from a passive observer seeking an end to an active participant who finds a new reason to exist. A pivotal moment occurs when she loses her memories and regresses to a frightened slave girl. Lelouch, feeling responsible, protects her, demonstrating his genuine care for her person, not just her power. Upon having her memories restored by the spirit of Lelouch's mother, Marianne, she chooses to side with Lelouch against Charles and Marianne's plan to merge all of humanity into a collective unconscious, solidifying her commitment to his path.

Beyond her immortality, C.C.s notable abilities include granting Geass to others, making her the source of Lelouch's power. She is also completely immune to the effects of any Geass used against her. She can telepathically communicate with certain individuals, notably with Marianne's disembodied consciousness, and can project powerful, chaotic hallucinations into the minds of others, as she demonstrated against Suzaku Kururugi. Her combat skills are formidable; she is a capable leader and a highly skilled Knightmare pilot. In the concluding events of the Zero Requiem, her role as Lelouch's protector is fully realized. As Suzaku notes, she becomes his shield, defending him from his enemies. In the epilogue, following Lelouch's planned assassination, she is seen traveling alone. Her final reflections reveal that Lelouch has proven her long-held belief wrong: that Geass brings only loneliness. Having finally expressed her true feelings for him, her long search for meaning appears to come to a close.