for more information see: Joan of Arc
She is first seen during the flash back shown by C.C. to Suzaku, Joan of Arc appear during C.C. flashback during the Hundred Years' War against the Kingdom of France and England (the precursor of the Holy Empire of Britannia) she and her Man-at-arm is killing everyone inside a church bearing a Geass sigil while proclaiming to C.C. and England that she is protected by the grace of God. She is also responsible for giving C.C. the Scar on her left side of the body. While it is not shown, but she is also responsible for killing innocent children while labeling C.C. as a Witch. When she was about to be burned alive at the stake she used every bit of her power to curse C.C. by giving her Immortality.
Unlike her real world counterpart, this Joan of Arc won't hesitate killing innocent and commit sacrilegious act inside a church giving her the name the Witch of Orléans and when she was burned at the stake, the crowd watching her was cheering for her death. Even when she was burning alive, she still managed to curse and laugh maniacally at C.C. saying that she will be the new witch.