The 98th Emperor of the Holy Empire of Britannia.
He has the Geass power to alter memories, and he took away Lelouch's memories as Zero.
He is the twin brother of the mysterious boy, "V.V."
During Lelouch's mission to destroy the Geass Organization, he takes away V.V.'s "Code" and becomes immortal.
He was thought to have been killed in the battle with Lelouch that took place in C's World, but he survived. He uses the Sword of Akasha in the ruins at Kamine Island and attempts to recreate a "world with no lies," the return of all mankind to enter the collective unconscious. The capture of C.C. and the attempt to make Britannia control the world was all for this purpose.
But the one who stood before him was his 'loving son,' Lelouch. The Emperor had the power to rule the world and immortality, but Lelouch cast a Geass on the collective unconscious. That world was then nullified, and the Emperor died.