Harry Potter/Vampire Knight Crossover
Potterverse moved up a decade. First Wizarding war ended on October 31st, 1991
It was assumed that Kaien ate his twin in the womb, but it was all untrue. Kaori Cross was born weak, but still alive. At 16, when their parents died on a mission, Kaien took his little sister to England, where the was no Hunter Association, and sealed her away in a place he thought was safe, where her growth would be frozen until the medicine needed to keep her alive could be created.** And Kaien was right, it was safe, for about two and a half centuries***, but on July 31st 1990, when they escaped from Voldemort for the third time, James and Lily Potter stumbled across the sealed and in stasis body of a still sixteen year old Kaori.
Having learned months earlier that they couldn't have children because of a curse cast by a Death Eater, they eagerly broke the seal, unknowing of the effects it would have on the trained Vampire Hunter.**** When Kaori woke up, she immediately collapsed, having not been exposed to the environment for two and a half centuries. Panicked, James and Lily apparated her to the boundaries of Hogwarts, where Lily sent a Patronus to Dumbledore and Madam Pomphrey. After the two met them halfway, Kaori was taken to the hospital wing and Lily and James went with Dumbledore to his office so they could tell him what happened, and that they wanted to adopt her.
Dumbledore accepted what they told him, and in return told them that if they wanted to blood adopt Kaori, he would have to cast a spell that was rarely used, which revert Kaori to a newborn, because a blood adoption doesn't work if the child in question was over a year old. It also wouldn't give her magic, as Kaori was a non-magical child. James and Lily didn't care.
So they spell was cast and the blood adoption went through, causing a hazel eyed blonde Kaori Cross to become a green eyed black haired Hazel Potter. Her memories were also suppressed, so people didn't suspect that she wasn't James and Lily's biological child.
HP canon events happen, but the Weasley Twins became her friends in place of Ron and Hermione, Kaori/Hazel became sick regularly, the blood of Lily and James not enough to completely erase what being born a Hunter's twin had done to her.
At Sirius' will reading, Kaori/Hazel was taken aside by the Goblins to tell her the last part of the Will, which Sirius had said was only allowed to be said to her. It told her that she wasn't James and Lily's biological child. Kaori/Hazel asked the Goblins if she could take a heritage test to confirm this, as she wanted to be sure. They agreed, for a fee, and she discovered who she really was. After a freak out about her age, Kaori asked if the Goblins could reverse the blood adoption, unbind her memories and find her brother. Once again, for a fee, they agreed and Kaori found herself in her previous body,
Kaori/Hazel didn't actually get magic, but the Horcrux from Voldemort allowed her to imitate magic until she had the blood adoption revoked, upon which her Hunter magic, which had also been suppressed, purged in from her body during the painful process of transforming back into the appearance of what she looked like when she was sealed away
*Don't know much about Hunter Magic, so use Bleach Kido
** Before Kaien sealed her away, Kaori would regularly have seizures and fits because her body had a lot of trouble adapting to not having the strength to keep her alive. Unlike Ichiru, Kaori did not have medicine readily available to her, the only thing that kept her alive was her regular use of Hunter Magic.
***Kaien never had the chance to go back for her because he was watched too much when his reputation as the 'Fangless Vampire' came about. He didn't want to risk her dying before she could really live.
*** Kaori was trained by her parents and got experience with her brother, but the HA were unaware she was still alive, as they assumed that Kaien had absorbed her because of his overwhelming power. Kaien absorbed her strength, but she could still use Hunter magic.