Um this one is kind of tough. I mean we don't even know her real name and we are 26 episodes into the series.
C.C. is an immortal youth and has lived for hundreds of years. She constantly refers to herself as a witch, due to the fact she was more than likely hunted during the witch trial eras. She has been burned alive, tortured, crushed in the ocean, shot several times, but you can't kill her. She entered into a contract with Lelouch which we know nothing about only that she will tell Lelouch eventually and she must keep him alive.
Her attitude is arrogant, crude, critical, but also supportive at times, and is emotional when her humanity is discussed. She also loves pizza. It is also believed she can speak to the dead, and with others like her such as V.V. telepathically.
In 2006 she entered into a different contract with an asian named Mao. However, she eventually ditched him when he wasn't able to fulfill the contract.