Sirius B

Since before you were born your family agreed to deny the existence of Sirius Black, it had never been a source of pride and he was in Azkaban at that time and therefore they did not want you to even know of the existence of your father's brother, Regulus Black, who also contributed to hiding the existence of his older brother Sirius Black.

You grew up with the idea that your father was an only child, to tell the truth you never questioned it, it wasn't something you cared to verify or something that directly affected you, so you lived without thinking much about the matter until you were 13 years old.

You were on vacation in your room, reading a book in bed before going to sleep, when suddenly something started scratching at the window, trying to open it. You immediately thought of your pet and indifferently opened the window a crack. You calmly returned to your bed until you saw a grown man enter your room.
Obviously you were alarmed, asking who that man was, why he was in your room, what his intentions were, etc.

"I'm Sirius Black. Who are you? "

The man asked, confused. He looked quite exhausted and sleepless; it was clear he hadn't been doing well for years.