From the Anime Code Geass
C.C. is immortal, healing from any wound (implied to include beheading) with enough time. She also doesn't age. Apart from her immortality, C.C. has the ability to bestow people with the power of Geass, which manifests differently in each person but generally carries the ability to affect the minds of those it is used on in some way. A Geass sigil appears on her forehead whenever she manifests this power, and she has a similar scar on her left breast. Her powers caught the interest of Britannia, who imprisoned her for experimentation purposes.
In the first season finale, C.C. triggers a trap set by V.V., causing herself and Lelouch to be submerged in a shock image sequence similar to the one she used on Suzaku. Through this, Lelouch sees memories of her past, including repeated "deaths". One shows C.C. attempting to escape from a World War I-esque battlefield, only to be shot in the forehead. Further memories show her being persecuted as a witch and suffering various forms of torture and/or execution throughout history and various time periods, including burning at the stake, guillotining, and the iron maiden. This would suggest a minimum lifespan of at least seven centuries. In a special feature included in the series' fifth DVD, she and Lelouch discuss Washington's Rebellion, during which she speaks of participants such as George Washington and Benjamin Franklin as if she had been close friends with both. In the past, she appears to have observed Suzaku and Lelouch when they were both children, having even interacted with Lelouch at one point, though he doesn't remember their meeting.
Her individual experiences have led C.C. to become a detached and lonely individual, with little care for even those she has entered into contracts with. She tells Lelouch that she has been alive so long that she can no longer remember who loves her and who hates her. Due to her nature and history, she has been lonely as long as she can remember. Lelouch replies, "You're not alone. We are partners. If you are a witch, then I'll become a demon." Lelouch is possibly the first person that has ever truly cared for her and it appears that she reciprocates at least some of these feelings, as she unexpectedly kisses him after their memories are shared. However she claims that she has been selfish, keeping Lelouch alive just so that she could be able to die. In episode 23 of the second season, Suzaku asks her to be Lelouch's shield, defending him from his enemies while Suzaku strikes down those enemies as Lelouch's sword.
C.C. is willful almost to a fault, usually doing whatever she wants regardless of whether or not it inconveniences others, particularly with Lelouch. In episode 5, she purposely make use of her contract to make it sound like it was an engagement in front of Nunnally and in one Sound Episode, she said to Nunnally that both she and Lelouch are 'busy' making Nunnally misunderstood.
When left alone, she has sometimes spoken to a previously unknown person through means unknown, which, in episode 23, was revealed to be Marianne, Lelouch's deceased mother, although in episode 19 it appears that she is talking to V.V. as well. According to the DVD Magazine 02, she is talking to more than one person when she seems to be talking to herself.
Before becoming immortal, C.C.'s Geass, as contracted with a nun during apparently medieval times, granted her the power "to be loved," which allowed her to make anyone around her love her. She originally started with a Geass symbol in her left eye, but as she matured, she eventually developed another in her right, and inferredly became unable to control it, causing everyone to love her. After C.C. had became bored by the tranced society around her, the nun who had bound the Geass contract to her shifted her immortality into C.C., as signified by the Geass sigil on her forehead. The nun died in the process, and as part of the exchange, C.C. became immune to the effects of Geass while losing her original.
<videogallery> Video:Code Geass - Code name CC </videogallery>