C.C. is a Finnish slave girl who received a Geass as part of a medieval weapons program and survived long enough to become an immortal.
Marianne is a French knight who was so powerful she, a mere commoner, became the first ever female Knight of One and then became a Queen.
V.V. and Charles are twins, born into Britannia at a time when the heirs to the throne were killing each other. V.V., who had the strongest Geass potential, was given a Geass by C.C. at birth and he became an immoral by age 9. V.V. then contracted himself with his own brother, who used his Geass to take control of Britannia and steer it out of chaos.
Mao was a member of the royal family of China, the last male in the Han Dynasty, who was abducted by C.C. during the Romance of the Three Kingdoms and turned into a Geass user. However, Mao wasn't mentally capable of maintaining his Geass and was driven insane by it as it aged in power, so C.C. was forced to abandon him.
C.C. then joined the Geass Cult, which had been established by Britannia as a means for researching the origin and limits of Geass. After Marianne was assassinated, Charles wanted to perform experiments on C.C. (who lost all her status and connection to the throne) so she fled to Japan, where Marianne's children were sent. She was a guest of Kirihara Taizo and watched over Lelouch and Suzaku as children, though neither remember this.
It was at this point that C.C. gave Nunnally a Geass, one that allows her limited perception into the future and one that doesn't require physical vision to use. In the eight years since the fall of Japan, Nunnally's Geass has evolved in both of her eyes, but none can see the sigils because she always has her eyes closed.