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. However, there was a secret she was keeping from the Doctor...

Behind the Scenes
Big Finish Productions have not announced whether or not Amy will continue on as the Fifth Doctor's companion after The Key 2 Time - The Chaos Pool.