You
You run down the corridor, expecting something different. Expecting that perhaps he won't be kissing that girl he left the room with only a few minutes ago.
You open the door with a quick turn of the doorknob and find that he's doing exactly as you feared: cheating on you.
He stops and realises what's happened.
You can feel tears burning up in your face, you run back out of the room. You can't look weak, not now.
You try and run the way you came, but you can hear him calling out your name, trying to get you to come back.
You won't stop, you won't ever talk to him ever again, and that's for sure.
You're going to do the exact same thing he's done to you. You're going to play with his heart until you've finally got the strength to break it.
Why?
Why should you sit down and take it?
Why should you just watch him being happy?
Why shouldn't you get your own back on him, after all that he's done?
You won't. You're the type of person who likes their revenge to be served cold. Wait a few months, and then do it. Mess with his head.
Bake his soul, fry his heart and poach his head, it wouldn't matter. Just mess with his head, and hope he finally gets it.
That's right.
James Sirius Potter is going to be just like you now by the time you've finished.
Broken hearted and alone.