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, like her "twin sister" Zara, existed to find the different segments to the Key to Time to stop its decay from when the Doctor had recklessly put it together before. The Fifth Doctor was brought to a rainy planet at the end of time, finding a compass which led him to Amy, then just a few minutes old and not yet having a name. Telling him this was a segment to the Key to Time, she asked if he could help her collect two more segments. She turned the first segment into its original form and put it into a satchel leading to another universe.

Arriving on Mars in the distant past, the Doctor decided to name the Tracer "Amy", thinking she had agreed upon the name. Amy realised the segment of the Key was disguised as a capstone on the pyramid in the town. The Doctor and Amy were arrested and taken to Magistrate Isskar, accused of being thieves by Isskar's beadles. They were let go with a warning not to approach the pyramid. Understanding but disobeying the warning, inside the pyramid, the Doctor and Amy met another tracer whom already had one of the segments of the Key in her possession, along her assistant in her quest, Harmonious 14 Zink. The Tracer agreed to the Doctor's suggestion of "Zara", "just to shut you up". At the top of the pyramid, Zara touched the segment and turned it back into a crystal. This caused a gravity well to appear, causing massive changes to Mars' environment. Zara left with the segment using Zink's time ring. The Doctor, Amy and Zink jumped on board an approaching ship, and the Doctor piloted it to safety. Unable to help the catastrophe inflicted on Mars, the Doctor and Amy left in the TARDIS for their next destination.

The Doctor and Amy arrived inside a castle on the ring world of Safeplace about 16,000 years later. At a castle there, there was a succession dispute between the Valdigians, Lady Mesca and Wembik. The Doctor concocted an anti-venom for Mesca, whom Wembik had poisoned because she refused to allow Mesca's son to become king.

Meanwhile, the Ice Warriors, led by Isskar, now an Ice Lord, were guided to Safeplace by Zara to find the next segment, and began fighting the Valdigians. When the Ice Warriors were repelled, Amy worked out a compromise between Mesca and Wembik — they agreed to be married to one another, so that no matter whose son became king, he would also be the son of the other one. While the Doctor asked Isskar to speak to him, his men brought Amy outside the castle grounds.

Zara spent generations manipulating the Valdigians as well as the Ice Warriors. She realised the castle was the next segment and turned the segment into a crystal, trapping the Doctor, the TARDIS, the Valdigians and Isskar inside. She tried using the power of the segments to take Amy's segment from her satchel. Instead, Zara blew up her satchel, and the Safeplace segment ended up in Amy's possession; Amy assumed what happened was the segments "just balanced out". While Zara left with the time ring, Amy turned the segment back into a castle, allowing the Doctor, the Ice Warriors and the Valdigians to escape; the Ice Warriors took the TARDIS outside. Amy then turned it back into a crystal and placed it into her satchel.

The Ice Warriors took the Doctor and Amy onto Isskar's ship to take them a tribunal; Isskar confiscated the segments and sent the TARDIS overboard while the ship was in hyperspace in the event of the Doctor and Amy's escape. Zara, being unable to detect Amy's segments, travelled back in time with the time ring, having "wrecked the controls" of the ship. The ship was on a collision course with the red giant, Leboon. Isskar showed Amy her segments, hidden outside time inside a strongbox, which Amy opened. When the last escape pod was taken, leaving the Doctor and Amy behind, the Doctor tried fixing the ship, before the Black Guardian intervened, telling the Doctor that he hoped the Doctor could explain the situation. (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 II 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)

When they escaped the village, Zara and Abby were captured by Marek, who used time technology to escape the Sphere before it's destruction. While in flight, they were captured by Kreekpolt. Kreekpolt wanted to use the twins powers to heal his daughter, who was fatal ill, but they were unable to do so. However, they suggested they go to the Grace, who would have the power to do it. At gunpoint, they were led to the Chaos Pool. When they arrived, the Grace granted the tracer twins immortality, and they could only die when they wanted to. After this, Abby, Zara and Marek departed from the Chaos Pool, hoping to start a new life together, raising the baby after its birth. (AUDIO: The End)

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.