Not much is known about False Ciri's past or even her real name. She was of Cintran nobility before her parents were killed, likely during the first Northern War, leaving her an orphan, and went through several refugee camps before finally ending up in Brugge, where she worked by measuring out cloth. When Codringher and Fenn was tasked by Geralt to learn why a sorcerer named Rience was trying to so hard to get his hands on Ciri, Codringher learned of a girl, similar in age and appearance to Ciri, and tried to persuade Geralt to use her as a decoy to get Rience to back off. However, Geralt refused to do so and the matter was dropped.
The night the Thanedd coup started, Rience sent Schirrú to deal with the detective agency the same night and the half-elf, along with another accomplice, Nazarian, found the notes on the lookalike. They then met up with Rience on July 3rd, a few days after the failed attempt to get the real Ciri, and after some discussion, went to Brugge the next day and kidnapped the girl before handing her off to Pitcairn Godyvron in Verden.
By July 10th, she had been transported to Nilfgaard and put under CountessStella Congreve's care. Later that month, EmperorEmhyr var Emreis formally welcomed her and gave her the title of Suzerain of Attre and Abb Yarra (one of the real Ciri's titles). However, he knew immediately that she wasn't the real Ciri, as the real one was his daughter, and thus sent False Ciri to Darn Rowan, one of the farthest points from the capital and thus out of sight while the search for the real Ciri continued.
At Darn Rowan Castle, Stella continued to educate the young woman when, 6 months after Emhyr formally met her and sent her away, he came to visit while a portrait of her was being painted by Robin Anderida. As it was believed they were close to getting the real Ciri, something had to be done with False Ciri. After he asked if there was anything she wished for, and receiving a no, the emperor then asked for her real name, but False Ciri refused, stating her own name meant she was a nobody while as Cirilla she was important. After reassuring her no harm would come to her, the emperor once more asked if there was anything she wished for and this time she responded that she wished to stay there, in Darn Rowan with the countess. Though the emperor personally thought it was a mistake on her part he agreed to it.
"I gave my word, ... and I shall keep it. I think, nonetheless, that you've made a bad choice. You gave voice to the wrong wish. Were you to change your mind ..." "I shall not, ... Why should I? I've chosen Lady Stella, I've chosen things I have known so little of in my life ... A home, warmth, goodness ... kindness. You can't make a mistake by choosing something like that."
A short time later, she and Stella visited the city of Nilfgaard, after Emhyr had found and talked with the real Ciri and had let her go. Emhyr then decided to marry False Ciri, making her empress, though it was only to secure Cintra as part of the Empire.
Though she never features in the release version of The Witcher 3: Wild Hunt, "False Ciri" was mentioned in leaked materials from 2014. She was to appear on Skellige and use another name - "Becca" (the same as the second queen of Cintra). It was not explained whether or not this was her real name or just another alias.
Ironically, centuries later, the only known portrait left of Ciri is actually of False Ciri.
Though it's insinuated that Emhyr started to have feelings for False Ciri, it's never actually specified if he did fall in love with her.