Abby

Abby, also known as Amy in certain periods of her life, was a tracer in human form. She travelled with the Fifth Doctor as his companion during his second search for the Key to Time.

As Amy
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 the compass was a segment to the Key to Time, she asked if he could help her collect two more segments.

Arriving on Mars in the distant past, the Doctor decided to name the Tracer "Amy", thinking she had agreed upon the name.

Taking possession of the segments Zara had, they assembled the key, before the Black Guardian intervened, telling the Doctor that he hoped he 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)

As Abby
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. While under the influence of the Sphere, she destroyed it upon her and Zara's escape. (AUDIO: The Sphere)

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 its destruction. While in flight, they were captured by Kreekpolt. Kreekpolt wanted to use the twins powers to heal his daughter, who was fatally 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)

Zara and Abby changed time to make a city survived being killed, but lost Marek in the process. (AUDIO: The Line)

She wanted to rescue a crashed ship. After Zara went missing she wanted to sacrifice her life so that the inhabitants of a village she stayed at could escape. (AUDIO: The Flood)

She met with someone who said she was Zara's daughter. She was still naive about some parts of sex and asked a lot of awkward questions. (AUDIO: The Dark)

She tried to hook Zara up with Kurt. She later lost her memories. She went after Zara when she went into a burning building. (AUDIO: The Edge)

She went to medieval England and tried to stop a battle. (AUDIO: The Battle)

She wanted to go back to the Archive and talk to Chi about the day of her death. On arrival she was arrested for murder. (AUDIO: The Archive)

She came and saw Brondle and Wing, here she met Marek again. She wanted to save the planet from the sun which would implode. Abby and Zara decided to die but the Grace told them that they were an experiment into balance and decided to remove their powers instead of them dying. Abby decided to be known as Amy again as she felt she was no longer Abby. (AUDIO: Consequences)

Three centuries later, Amy was in hospital because she fell out of a tree. Zara told her about Pool and their plans to stop a nuclear fusion reactor. Knowing she was dying, Amy asked Zara to get all the family together. She came along to help Zara persuade the guards. She got annoyed when Dodyk wanted to stop them. She helped Zara to stop the reactor. They consumed the energy from the blast and the fuel and became young again. (AUDIO: The Bomb)

Abby went with Zara to see a rare snake but they soon got caught up in the war that was waging across the planet. She suggested to Onora Cormorant to stop the war by surrendering and got her and Slink to try and talk. Abby discovered from Cormorant where Zara was being kept. Later she realised how to stop the war when she ordered Kronchev to kill everyone on the moon. When that didn't work she set up another plan to stop them. (AUDIO: The Room)

Abby pretended to be a doctor and sometimes helped Zara in surgery. She found it hard when a set of children died. Abby later went on a date with Chaff but lost him when he slept around. Pool arrived at the hospital to tell Abby and Zara that the Grace needed them. (AUDIO: The Ward)

Pool took them to the end of time to a ballroom, trying to get answers on what was happening from Oblong and Triangle. Abby thought that Pool was sent by the Grace to judge their balance. She took Pool to see them before they got their powers back. She got the dancers to decide what they wanted to do. (AUDIO: The Dance)

Personality
Abby liked her tea with milk substitute and four sugars. (AUDIO: The People Made of Smoke)

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 was to avoid potential confusion with Amy Pond.
 * Abby and Zara were at one stage planned to feature in the audio story Relative Time, part of The Legacy of Time box set. Ultimately, however, their place was taken by Jenny. (BFX: The Legacy of Time: Behind the Scenes)