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 . Her powers caught the interest of Britannia, who imprisoned her for experimentation purposes.
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 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.
C.C. is willful almost to a fault, usually doing whatever she wants regardless of whether or not it inconveniences others.