Zara

Zara, later Joy, was a tracer in human form.

Biography
Zara was created by the Grace, powerful beings who created the Key to Time, to prevent the completion of the Key. Like her twin sister, Amy, she existed to find the segments of the Key to Time. (AUDIO: The Judgement of Isskar)

During her journey, Zara was taken prisoner with the Seventh Doctor's companion Ace on the planet of Erratoon. She escaped with the help of Harmonious 14 Zink. (AUDIO: The Prisoner's Dilemma)

Zara and Zink would encounter the Fifth Doctor and, her twin sister, Amy on the planet Mars. Zara would soon cause the collapse of the Martian society after finding a segment of the Key to Time. (AUDIO: The Judgement of Isskar)

After being freed by the Grace, Zara made her way to the Sphere. On the Sphere, Zara met Marek, who got her pregnant. Abby, formerly Amy, followed Zara to bring her back to Pargrave, a soldier whom Zara had fallen in love with. They used their powers to earn enough money to escape from the Sphere. However, in a rage, Abby destroyed the Sphere. (AUDIO: The Sphere)

After escaping the Sphere, they were taken into custody and accused of killing the loved ones of the people from a mysterious village called Compton. During an escape attempt Zara was left behind in their jail cell. Zara was threatened with death until the angry mob believed that Abby was also killed. Abby escaped and found herself slightly outside of the village only a week into the future and the village destroyed. It turned out that Compton was destroyed by a meteor strike, and that the tracer twins had arrived the moment that the meteor hit. The people in the village were echoes of the dead, and Abby and Zara escaped before they were destroyed by the meteor. (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 when it is born. (AUDIO: The End)