Abby

A tracer in Human form, Amy was a companion to the the Doctor in his search for the Key to Time.

Biography
Amy, like her "twin sister" Zara existed to find the different segments to the Key to Time. When she first met the Doctor, she was just a few minutes old, and recruited his help in tracking the six pieces of the Key to Time, making her the first companion who makes the Doctor her companion. (BFA: The Key 2 Time - The Judgement of Isskar)

Her travels with the Doctor turned her into a human being, one with a strong sense of right and wrong. She became a very clever and resourceful young woman, for example saving the life of Romana whilst Zara and the Guardians wanted to convert her into the final segment of the Key. After the Key was destroyed by the Doctor, Amy was invited by Romana to Gallifrey to join to Academy to become a Time Lord. (BFA: The Key 2 Time - The Chaos Pool'')

Behind the Scenes
There has been some speculation that Amy could return at some point in the future.