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 for the majority of the story is daring to have a different way of life than Ebony. This pisses Ebony off greatly, making her give Britney the middle finger every time she sees her.
Britney once dated Vampire Potter before the story started, but the two broke up for reasons unknown. Her relationship with Vampire was also the reason Vampire broke up with Draco.
In Chapter 24, Ebony and a few other unnamed preps are forced to work after Professor Sinister had already dismissed the entirety of the class, showing that even some of the teachers, who typically should not display student favoritism, have a negative opinion of her.
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, in her first truly evil action. Ebony tortured her with a 'crocious curse' and then left her to her own devices. She is not seen afterwards.