Britney is a minor character that appears in My Immortal.
Britney is Ebony's arch rival from the house of Griffindoor, but the only thing Britney ever does wrong in the story is daring to have a different way of life than Ebony. This pisses Ebony off, making her give Britney the middle finger every time she sees her.
Britney dated Vampire Potter before the story started, but broke up for reasons unknown. Her relationship with Vampire was also the reason Vampire broke up with Draco.
Near the end of My Immortal, it is revealed Britney allied with Voldemort and Da Death Deelers and freed Snap, Loopin and Hargrid from Azerbaijan. Ebony tortured her with a 'crocious curse' and then left her to her own devices. She is not seen afterwards.