This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
"That I'm a soulless murder machine?"
"That you are something lovingly and deliberately created, like Dahj oncidium."
"You're telling me that I'm not real."
"No, I'm not. If you are who I think you are... You are dear to me in ways that you can't understand."
Dahj believed that she was Human; she apparently had a Human-like physiology, being capable of drinking, suffering lacerations, bleeding, and having lacerations healed with a dermal regenerator. Dahj remembered being raised in Seattle, and her father, a xenobotanist, developing a new breed of orchid and naming it after her: Orchidaceae Dahj oncidium.
In 2399, she resided in an apartment in the Greater Boston area, and had just been accepted to the Daystrom Institute as a research fellow in artificial intelligence and quantum consciousness. (PIC: "Remembrance") Among her credentials supporting her application for the fellowship were transcripts from education at R3SA on Regulus; Dr. Agnes Jurati later characterized her resume as making her a "perfect candidate" for this fellowship, but later discovered that there was no direct record of her attendance, and that her identity appeared to have been created about three years earlier, around 2396, possibly by Bruce Maddox. (PIC: "Maps and Legends")
While celebrating this acceptance with her boyfriend, Caler, she was attacked by Romulan assassins. Activating previously unknown combat skills, she killed her attackers and fled in shock. Seeing visions of Jean-Luc Picard, and witnessing part of his Federation News Network interview on the anniversary of the Romulan supernova, she sought out him out at Château Picard in France, where she expressed her uncertainty to Picard about what was happening to her and her feeling that she was safe with him.
Although Picard allowed Dahj to stay at his home that night, she departed before morning, believing she was putting him at risk. She contacted what she believed to be her mother, who urged her to return to Picard, despite Dahj never having mentioned his name. She became suspicious, and, using android-like skill and speed, hacked into surveillance systems to locate Picard at the Starfleet Museum Archive, shortly after Picard had confirmed that Dahj's image appeared in a painting made by Data in 2369 entitled Daughter. Around the same time, she noticed heightened abilities, such as the ability to hear conversations at long distances, and became concerned that she was suffering from schizophrenia or head trauma.
After revealing to Dahj his suspicions that she was an android connected somehow with Data, Dahj and Picard were attacked by Romulan assassins. Dahj protected Picard and fought the assassins, one of whom spat a corrosive substance on her and the rifle she had taken. Shortly after the rifle exploded, destroying Dahj. There was no record of her apparent death in any surveillance footage.
Dahj possessed a necklace which was (at least in her recollection) given to her by her father, with a pendant in the form of two interlocking circles. After meeting Picard, she left the necklace on a table they were sitting at, having not put it back on after showing it to him. After Dahj's apparent death, cyberneticist Dr. Agnes Jurati recognized the pendant as a symbol of fractal neuronic cloning. Jurati speculated that Dahj was created via that technique, a radical idea proposed by Bruce Maddox that suggested that a positronic brain could be created entirely from a single positronic neuron, in this case, from the late Lieutenant Commander Data. Such processes would produce two twin androids; Dahj's twin, Soji Asha, appearing totally identical in appearance, was, at the same time, working at the Romulan Reclamation Site. (PIC: "Remembrance")
Soji kept a picture of her and Dahj by her bedside, and it was one of the objects she scanned with a Romulan dating device. According to that device, the image was only 37 months old. (PIC: "The Impossible Box")