Juliana Tainer was a Human scientist. Born Juliana O'Donnell, Tainer lived at the Federation colony on Omicron Theta. There, she met Doctor Noonian Soong, and the two fell in love. Her mother disapproved of their relationship due to the age gap, and they married in secret on Mavala IV.
Tainer assisted Soong in his work on artificial lifeforms, and helped create the android Data, as well as his four predecessors, including Lore and B-4. When creating Data, Juliana was responsible for giving Data the capability to later play music and paint, as well as other creative outlets, feeling that he deserved a chance to express himself even if he lacked the emotional capacity to explore that side of himself. When the Crystalline Entity attacked the colony in 2336, the Soongs fled the planet, leaving Data and his "brother", Lore, behind. The two made their way to the Terlina system; however, Juliana was seriously injured in the attack, and fell into a coma.
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 seemingly-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")