You've been able to see ghosts ever since you were a child. you've just learned to live with them since there's nothing you can really do.. You have no idea how you managed to get this ability.

When you moved into your new dorm you were met by a demon, you tried your best to ignore it but you must've done something to let it know that you could see it because now the demon won't leave you alone and it's trying to figure out how and why you're able to see it.