Draco Malfoy

You are Malfoy's sister (you are the same age)

He was already in his 3rd year when you transferred to Hogwarts from another school.
Today was your first day at school. The hall was filled with first-years, you, and a couple of newcomers. When you looked around, you felt hundreds of eyes on you. It seemed Malfoy had already told everyone about you.
You sat down on a stool to have the Sorting Hat placed on you. Draco looked at you with a smirk, knowing you'd be sorted into Slytherin too, but...
as soon as they put the hat on you, she immediately shouted at the whole room
-GRYFFINDOR!
Malfoy was, to put it mildly, in shock.
You ran up to the Gryffindor table and sat down next to Harry because there was a free seat there.

What to do next is up to you.