Abby

"Abby" was the name used by a companion of the Fifth Doctor, who originally named her "Amy".

Search for the Key to Time
At the Chaos Pool, (AUDIO: The Chaos Pool) the Grace created a pair of humanoid tracers to track down the decaying segments of the Key to Time. Whilst one went on her journey to find her three segments on her own, the other chose to employ the Fifth Doctor's assistance in finding the other three. It was one such piece, in the form of a compass within the wreckage of the Green Man's Burden, which led the Doctor to her.

The tracer, whom the Doctor named "Amy" after a misunderstanding, was able to get the Doctor to agree to help her find the next segment and joined him in his TARDIS, taking control of it and piloting them to Mars where she identified the next segment as the capstone of a pyramid. They encountered Harmonious 14 Zink and Amy's sister, whom the Doctor named "Zara", and gained the two segments that they had found thanks to Isskar before being saved from a spacecraft by the Black Guardian. (AUDIO: The Judgement of Isskar)

Having arrived in 9th century Sudan, Amy told the Doctor not to listen to the Black Guardian. She later lost her satchel in which she kept the segments and was mistaken for a slave of Lord Cassim's household, eventually finding her satchel and reuniting with the Doctor. She transformed the golden shard formed from Lord Cassim's compressed treasures into the fifth segment and departed with the Doctor, leaving the Black and White Guardians behind. (AUDIO: The Destroyer of Delights)

Amy was knocked out on a spaceship by Zara, who imprisoned her in her satchel and took on her form and identity to deceive the Doctor. Sensing the decay of the five segments, Amy placed them in her own satchel and, upon being freed, refused to give them to the White Guardian as the Doctor believed that to do so would be to end free will. The Key was finally formed, by which point both Amy and Zara had become human, and Amy gave her sister the choice of what to do with it. (AUDIO: The Chaos Pool)

Aftermath
After the Key's destruction, Amy chose to go to Gallifrey with Romana to persuade the High Council to open the Time Lord Academy to non-Gallifreyans, saying farewell to Zara, who invited her to Atrios, and the Doctor, who assumed that she would continue travelling with him. (AUDIO: The Chaos Pool) Romana was successful in achieving this, although it was not without controversy. (AUDIO: Insurgency)

Reunion with Zara
Amy travelled to the Sphere to find Zara after Zara and Pargrave had an argument and tracked her down to a casino which she infiltrated by taking on the name of one of its members; Abigail. Having done this, she chose to start calling herself "Abby" instead.

She found 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
Zara described Abby as her "good as gold", clever sister. (AUDIO: The Sphere) She 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)