Juliana Tainer was a Soong-type android. She was a replica of the Human Juliana Tainer. The real Juliana died from injuries she received from an attack by the Crystalline Entity on Omicron Theta in 2338.
In a last attempt to preserve her memory, Soong built a new android, one indistinguishable from a Human, and transferred his wife's memories into it, using a synaptic scanning technique. Following Juliana's death, Soong activated the android, who remained unaware of her new state of being. Soong deliberately designed the android to be so, and programmed her to shut down in the unlikely event of her android nature being discovered. She was also programmed to eventually deactivate and self-destruct her positronic matrix, essentially giving her an eventual "natural" death.
However, the relationship between the two soured when Soong became too focused on his work, and Juliana left Soong – something that Soong noted the 'original' Julianna would have done as well – and made her way to Atrea IV, where she met and married the Atrean scientist Pran Tainer, becoming his assistant. When the USS Enterprise-D arrived at Atrea IV in 2370 to assist in planetary core experiments, Data and Tainer were reunited, although Data was unaware of Tainer's contribution to his design, as his memories of those early days had been erased. Although Data was initially uncertain about how he should feel about Juliana's return to his life, a conversation with Geordi La Forge convinced him that he couldn't 'hide' away from Juliana simply because he was uncomfortable with the change she represented, prompting him to begin spending time with his mother. Although the two bonded over Data's creative outlets such as his violin playing - Data noting that he felt most Human when he was exploring his creative side -, there were still some awkward moments between them, including Juliana's revelation that she had actually been responsible for leaving Data behind on Omicron Theta because she was afraid of him turning out like Lore.
During the mission, Tainer was injured, and her true nature was discovered (Although Data noted that he had observed other clues about her nature earlier, such as her ability to perfectly play the same viola piece the same way on separate occasions, or her blinking pattern being based on the same pseudo-random sequence that Soong had used for his own). After viewing a hologram of Soong designed to explain what had happened, Data chose not to reveal Tainer's true nature to her, concluding that it was more important for her to live a happy life believing herself to be Human rather than take away from her the goal that he had sought all his life simply to give himself something he could share with her. Before she departed, Data told her that Soong had once told him that he only ever had one great love in his life, expressing his certainty that she was the woman in question. (TNG: "Inheritance")