C.C. does not even know how old she is; her existence on Earth has been for hundreds of years. She can remember, dimly, memories: being shot in the head during WWI; being persecuted as a witch; the Washington's rebellion; watching Lelouch and Suzaku as young children. It's due to her great age and the bitterness of experiences that makes C.C. wary of people. She states that she cannot even remember who loves her or who hates her.

Her abilities to bestow Geass, her regenerative abilities that basically render her immortal, and her psionic powers made her interesting to the Britannian government, particularly because they too wished to unlock the secret of immortality. C.C. was captured, gagged, bound, made completely powerless and shipped around under the guise of being a capsule of nerve gas.

That is how she meets Lelouch and bestows upon him the power of Geass. He and he alone is able to get closer to her than any other character: he alone knows her real name, has seen moments of her memories, knows of the stigmata under her breast. C.C. helps him on his quest to defeat and destroy the Britannian empire, though it isn't clear what her intents or purposes are. She acts in his steed when needed, and the Black Knights assume she is his mistress, particularly because they share the same bedroom.

(For those interested, she makes Lelouch sleep on the floor.)

Throughout the series, it appears she is talking to some unknown person, though it is eventually revealed to be Lelouch's deceased mother, the Empress Marianne. It is unclear how she maintains this connection.

C.C. adjusts easily to her life with Lelouch, integrating herself without question and even despite his refusal to allow her in. She sleeps in his bed, runs up his credit cards, and puts him in messes by randomly appearing on campus. However, it is undeniable that C.C. acts as some pillar of sanity for Lelouch, or at the very least something constant as he continues to change his world.

She does show some signs of tenderness, particularly when he saws, "If you are a witch, then I am a warlock." At the end of the series, C.C. is shown drifting to the bottom of the sea in a box, apparently destined to die.

It is here she says what may be her most important lines and may show what her intentions are:

"Humans exist to search for happiness. What he [Lelouch] was looking for is just that bit of happiness. It's not something special. The root of his actions - as a person, they are very tiny wishes that can be clearly seen in everyone. These dreams, these vows - who can deny them? Who has that right? However, no matter who made the wish, no matter what the wish is, one cannot bind the whole world to himself. ... he is blocked by the past he created - hate between people. Still, we should be thankful that humans desire happiness, for it is from despair that hope is born."