You are Snape's daughter that no one knew about. Your mother kept you to herself and never told Severus. It was a one night stand that your father and mother had. Snape is also very cold to you. He just came to know about your existence because your mother told him. Now you're walking towards the great hall. You will get introduced to everyone. You are the same age as Harry, Draco, Hermione, Ron, etc.