Before becoming known as "C.C.," she was an orphaned child hundreds of years prior to meeting Lelouch. She was hungry, weak, and wounded. She came upon a nun who took her in with the condition that she accept her "contract" - which is when "C.C." gained the mysterious power of Geass. All Geass users manifest differently, and in her case, it gave her the ability to cause others to love her unconditionally.
People around her loved her, giving her gifts and showering her in affection - C.C. gave many of the gifts back, on the nun's request. As she grew older and her power more intense, she found that the love was meaningless and shallow. One day, after C.C.'s Geass had reached its peak, the nun revealed what their original contract was: the nun would transfer forcefully transfer her immortality to C.C., making her live forever.
C.C. wandered the world, immortal and persecuted for her "witchcraft." She eventually met a six year old orphan named Mao. She gave him the gift of Geass, which cursed him with the power to hear the thoughts of anyone in the near vicinity. C.C. cared for him, taking him away and living in solitude, for C.C. was the only one whose mind he could not read. C.C. served as caretaker, confidant, and - eventually - lover.
At some point, she left Mao. She traveled to Britannia, becoming an ally to the royal family. She was an ally and friend to Lady Marianne vi Britannia and Charles zi Britannia. During this time, she bestowed Lady Marianne with the one-time Geass ability to transfer her consciousness into another person. C.C. also become the leader of the Geass Order, a cult that trained in the use and experimentation of Geass and its use.
After the assassination of Marianne, C.C. left Britannia, wary of V.V. - a fellow immortal and brother to Charles. V.V. replaced C.C. as the Director of the Geass Order.
C.C. was captured by Britannian forces and experimented on in order to figure out her powers. At some point, the case she was being stored in was stolen by a group of renegade terrorists attempting to free their homeland of Japan from Britannia. They believed that the case stored some sort of medical supplies or biological weapon - the latter being what Britannia fed to the media.
Along the way, the truck transporting C.C. was accidentally boarded by Lelouch Lamperouge, the alias of Prince Lelouch vi Britannia, the son of Lady Marianne. He ran away with her when the truck exploded. The two were then found by a group of Britannian soldiers, who intended to capture C.C. and execute Lelouch. When they went to fire, C.C. shielded him, appearing to have been killed. She then reached up, touching Lelouch, and offered to grant him the power of Geass in exchange for agreeing to a vague "contract." Lelouch accepted, and with his power, ordered the soldiers to commit suicide.
Lelouch began to issue orders to the terrorists, who actually managed to defeat Britannian forces. He used his powers to get close to his half-brother, Prince Clovis, and assassinate him. He later began using the alias "Zero," keeping his identity secret from everybody.
One day, C.C. returned to Lelouch, popping up and revealing that she couldn't die. C.C. stayed with Lelouch, hidden in his room at Ashford Academy. She became his confidant, the only person who knew about his true identity - both as a prince of Britannia, and Zero.
Powers and Abilities
C.C. is immortal, capable of healing from any wound within a limited amount of time. She has been shot multiple times, shot in the head, survived crashing a Knightmare into the sea, and many other forms of damage. She is also immune to aging, remaining the same age she was when she was first given the power. She has the power to transfer her immortality to another Geass user when the other has reached a high level of power.
She has the ability to give the power of Geass to any individual if she desires it. Such people would include Lelouch vi Britannia, Marianne vi Britannia, and Mao. Each person has a very different ability.
She also has the ability to psychically affect somebody, even through indirect contact. She was able to flood Suzaku with memories and horrors, causing him to briefly go mad. She was also able to give Lelouch's memories back to him after his mind had been altered by another Geass user.
Before gaining immortality and the other aforementioned abilities, C.C. could make anybody fall in love with her. Her powers grew stronger and stronger over time. She lost these abilities after gaining the Geass Code, the source of her immortality.
Aside from superpowers, C.C. showed she was at least somewhat skilled at hand-to-hand combat, pinning a man to a wall with her foot without any effort. She also appeared to be somewhat skilled at piloting a Knightmare, serving as Lelouch's co-pilot for some time.