Lucy Potter.

You're Harry's twin sister. You've always been in Harry's shadow, and you're tired of it. You're both in your fifth year, but you went to Beauxbatons, and he went to Hogwarts. You've decided to attend Hogwarts for the second half of your fifth year. You head to the Great Hall, wondering what house you'll be sorted into in a few minutes. As you continue to think, you bump into someone.
The body is like a brick wall. You look up.
To see who it was, to your surprise you see Draco Malfoy, Matteo Riddle, Theodore Nott, Lorenzo Berkshire, Blaise Zibini, the infamous seven boys.
Slytherin. The only reason your brother ever spoke to you was to complain about those boys. You stutter before walking away, leaving them confused as you head to the Great Hall.

You walk through the large doors of the Great Hall just as Dumbledore announces you as a new student. You hear quiet whispers and mutterings as you walk towards the Sorting Hat.

Random Student 1: "That must be Harry's sister."

Random Student 2: "She'll probably be in Gryffindor, like the rest of her family."

When you approach the sorting hat, Professor McGonagall says to you, "Welcome to Hogwarts," and then places the hat on your head.
The room falls silent as soon as the Sorting Hat begins to speak.
Sorting Hat: "Hmm, you're different from the rest of your family, but I can't figure out why... You're hard to place in any house. Smart as a Ravenclaw;
Brave as a Gryffindor and hardworking as a
Hufflepuff. But I must say, you're perfect for Slytherin: ambition, cunning, and power.
Yes... it would be wonderful in this house. They pushed you to do better:
To the best."
Everyone is silent, the hat is wasting its precious time.
Sorting Hat: "SLYTHERIN"
Everyone looks shocked
The Gryffindors look annoyed; jealous.
While the Slytherins look proud and excited, they all cheer as they walk towards the Slytherin table.