Lily Slytherin

Lily Dorea Slytherin (born Potter) was a half-blood witch and the biological daughter of Lily Evans Potter and James Potter. Her birth was foreshadowed by a prophecy that was commonly interpreted to state that either she or Neville Longbottom would vanquish Voldemort.

Early life
After Severus Snape told Voldemort about the half of the propehcy he had heard, Voldemort decided that Lily was the individual mentioned in the prophecy, as opposed to Neville. When the Potter family found this out, they were forced to go into hiding. Voldemort attempted to kill Lily to circumvent the prophecy when she was a year and three months old. During this attempt he murdered Lily's parents as they tried to protect her, but this unsuccessful attempt to kill Harry led to Voldemort's first downfall. This downfall marked the end of the First Wizarding War, and to Lily henceforth being known as the "Girl Who Lived", an alias that would later be modified to "The Girl Who Lived Twice".

One consequence of Lily's loving sacrifice was that her orphaned daughter was given to her Muggle aunt Petunia Dursley to be raised. While in her care she would be protected from Lord Voldemort, due to the Bond of Blood charm Lily had accidentally cast when she sacrificed herself. This powerful charm would protect her until she became of age, or no longer called home the house of a close relative of his mother.

Hogwarts years (1991 - 1997)
Lily was abused while living with the Dursleys, leading to her resenting Albus Dumbledore's decision to place her with her Muggle aunt and uncle when she learned about it in her first year at Hogwarts. As it turned out, these individuals were not the only individuals her mother had considered family and thus not the only individuals she could safely live with; she discovered that Severus Snape, had become her mother's magically adoptive brother through magic the two had performed while attending Hogwarts School of Witchcraft and Wizardry together. Desperate to not go back to the home in which she had been abused for ten years, she requested that Dumbledore allow her to live with her magical uncle. He refused on account of Snape's apparent attraction to the eleven-year-old who looked like his first love, although Lily was unaware why the request was rejected, interpreting it as only having been rejected because Dumbledore didn't want her fame to go to her head.

In response to the rejection of her request, Severus Snape informed her that there was an individual more powerful than Dumbledore who could help Lily. This individual was Voldemort, who had been restored to life by Bellatrix Lestrange less than a year after his "death" when she escaped from Azkaban and used one of his Horcruxes to bring him back to life.. Snape also informed her that due to the charm that was on her, Voldemort could not kill her or order one of his followers to kill her.

When Lily arrived at Malfoy Manor, the location at which Voldemort was temporarily staying, she and Voldemort discovered that he had accidentally made her a Horcrux when he'd attempted to kill her, due to the instability of his soul, explaining her scar and mysterious connection to him. Seeing no point to keeping her as his enemy with this realization, and presenting an alternative interpretation of the prophecy, where both individuals would live a half-life of fear without uniting, he adopted her as his daughter, making her the Princess of the Dark Side and Death Eaters.