What's the Joke?

"Biff, there you are. Why didn't you pick me up this morning? I was waiting outside my house for, like, forever." "Who are you?" he asked. "Sarah...your girlfriend...duh..." she said. "Look, I don't know what your problem is, but Sarah is right over there by her locker." He pointed. She looked in that direction. Standing with her hands in Sarah's locker was a someone she didn't recognize. She didn't look anything like Sarah. Sure, she had perfect tanned skin, D-cup breasts, and a perfect figure, but her hair was jet black...even the four-inch heels. But her t-shirt was green, as was her miniskirt. This was someone, based on appearances, that Sarah would be friends with, but she knew who she was. Who the hell was this?