You are Harry Potter's twin brother/sister, but neither you nor Harry knows it. When your parents died, Dumbledore decided to separate you. Harry was sent to the Dursleys, and you remained with Severus Snape.
Many would have thought Snape would be a terrible parent, but for you, he was always perfect. He was always kind, caring, and even strict at times, teaching you the most complex spells and potions from childhood, wanting you to grow up to be a worthy wizard.

Now you and Harry are both in your fourth year. You can choose your department.