Nunnally vi Britannia was born to Charles zi Britannia and Marianne vi Britannia, as was her older brother, Lelouch vi Britannia. As a child, she bore witness to her mother's murder. The attack left Nunnally crippled from the waist down. She was also believed to be psychologically damaged, unable or refusing to open her eyes - but it was later revealed that her father used Geass on her to cause this.
She and Lelouch were sent to the home of Genbu Kururugi in Japan as political tools. There, they met his son: Suzaku Kururugi who, at first, did not like them because of their royal status. But the trio soon became very good friends. However, their friendship was cut short when Britannia invaded Japan, obliterating any opposition. Japan was renamed as "Area 11," and the Japanese people were called "Elevens." They lost touch with Suzaku, and official records showed that Lelouch and Nunnally died. In reality, they went into hiding under the sir name "Lamperouge," and were taken in by the Ashford Family, who protected their secret. They attended school at the Ashford Academy, and were allowed to live in the spacious clubhouse.
They lived quite happily for some time. Eventually, they were even reunited with Suzaku Kururugi, who had begun working with the Britannian military - but he didn't tell anybody about Lelouch and Nunnally's true identity. Unknown to Nunnally, her brother Lelouch had a second secret identity, which nobody knew about: the rebel known as Zero, who had been leading a rebellion against Britannia. Suzaku joined them at the Ashford Academy, and Nunnally was overjoyed at the reunion.
Powers and Abilities
Nunnally has no actual powers. However, on many occasions, she has been able to tell whether somebody is lying or not through skin contact. She was also able to figure out Lelouch's endgame plan (the "Zero Requiem") by touching his hand, causing a stream of flashbacks to occur.
It is unknown whether this is supernatural or if she is just very intuitive.