Darkness is the sister of Diabolo, Crab and Goyle and is also a friend of Ebony. Her father was a vampire who raped her and her brothers, which is why she became a Satanist goff. She helps Ebony get ready to go back in time by helping her put on goff clothes and make-up. Supposedly, she's meant to be Ginny Weasley, but has no resemblance to her at all. Like the other minor goffs, she barely has any dialogue. She is referred to as Ginny in the final chapter, however.
In canon, Ginny married Harry after the Second Wizarding War and had three children with him. In this story, however, she doesn't appear to have a love interest, as Vampire is more interested in Ebony or Draco. Also, the notion of her father raping his children is absurd, as her parents Arthur and Molly were shown to be very loving, albeit protective.
Interestingly, Tara claims Darkness is 'Jenny', and Raven's real name happens to be Jenny/Jennie. If there wasn't already an avatar of Raven in this story (Willow), it could be assumed that Darkness would take that role.