Code Geass C.C. in First Season

C.C. is introduced in the first episode, where she emerges from a fake gas capsule seized by Japanese rebels. She saves Lelouch Lamperouge from the Britannian soldiers trying to recapture her, taking the shot intended for him, and grants him the power of Geass on the condition that he grant her greatest wish. Lelouch accepts and is granted the power to control people's actions. Since she appears to have died following this, he leaves her body behind.

She unexpectedly reappears in the fifth episode, casually folding origami with Nunnally. From this point on she passively oversees Lelouch's missions as Zero, wanting to keep him alive so he can fulfill his contract, often going as far as wearing his clothes and impersonating him when situations become life-threatening.

When C.C.'s existence is revealed to the Black Knights, she gains considerable unofficial standing, as she is seen by other members to be one of Zero's closest advisers, if not his top adviser. However, her willful attitude and lack of an official posting annoys some veteran members and causes confusion as to her role within the Black Knights. At one point, Tamaki goes so far as to accuse her of being a member only because she's Zero's lover, a claim which she coldly denies. When the Black Knights gain possession of the Knightmare Frame Gawain, she becomes its pilot while Lelouch operates the weapons.

In the season finale, she uses the Gawain to battle Jeremiah Gottwald's Siegfried so Lelouch can save his sister. She kisses Lelouch before she drags both the Siegfried and herself to the ocean floor to end the battle while narrating that the spark of Lelouch's actions was the core of human motivation: the desire for just a small amount of happiness. According to Yukana in a conference memo, C.C. was crushed by the water pressure, but being immortal, ultimately drifted to the surface and revived; it is stated that it was too gruesome to be shown.