Abby

Abby, originally (and later) Amy, was a tracer in human form. She travelled with the Fifth Doctor as his companion during his second search for the Key to Time.

Biography
Amy, short for Amethyst, like her "twin sister" Zara, existed to find the different segments to the Key to Time. When she met the Doctor, she was a few minutes old. She recruited his help to track the Key's six pieces. She was the first companion who made the Doctor her companion. (AUDIO: The Judgement of Isskar)

Her travels with the Doctor turned her into a human being with a strong sense of right and wrong. She became a very clever and resourceful young woman. She saved the life of Romana when Zara and the Guardians would have converted her into the final segment of the Key. After the Doctor destroyed the Key, Romana invited Amy to Gallifrey to join the the Academy and become a Time Lord. (AUDIO: The Chaos Pool)

Amy later followed Zara to a place called the Sphere. Zara tried to bring her back to Pargrave, a soldier whom Zara had fallen in love with. She changed her name to Abby to infiltrate the Sphere and find Zara, only to find out a major secret from her sister - Zara had fallen pregnant by a man called Marek. He had become Zara's protector. He taught Zara and Abby about the Sphere. Abby and Zara could not teleport out of the Sphere. They used their powers to earn enough money to escape from the Sphere. (AUDIO: The Sphere)

Abby and Zara finally escaped the Sphere, only to find more trouble. They found themselves in a village called Compton in 1912, surrounded by a mysterious fog. The villagers believed the women had killed their loved ones by making them vanish into thin air. The tracer twins were taken into custody. Abby escaped and found herself slightly outside of the village only a week into the future and the village destroyed. (AUDIO: The Fog)

Behind the scenes

 * Amy's name was changed to Abby for the Graceless series. At the beginning of The Sphere, Amy took the name of a woman registered in the Sphere. The out-of-universe reason for this development has not been revealed. However it was likely that the change was made to avoid confusion with Amy Pond.