Hermione Granger and Daphne Greengrass are the same person.

By using a time turner, Hermione lives a double life at Hogwarts, in order to fulfill her secret dream. First, she goes through the day as her actual self, a frizzy-haired muggleborn know-it-all. Then she changes her appearance, turns back time and goes through the same day again, this time as Daphne Greengrass, the blonde pureblood witch and slytherin's resident ice queen.

Astoria Greengrass goes along with it for her own amusement and because she always wanted to have a big sister. She covers for 'Daphne' whenever it's needed.