Dear Mr. Lupin,
My name is Septimus Snape. According to Harry Potter, you are quite reasonable and will not throw this away before hearing me out. A little known fact about the Snape family is our ability to travel between dimensions. Every generation, the heir of the family is sent to a parallel universe to spend our first 16 years. In between universes, there are many differences, most resulting from a change in history. For example, in the universe I grew up in, Voldemort was never brought into existence. Instead of Albus Dumbledore, Tom M. Riddle resides as Headmaster of Hogwarts.
I am sure at this point you are asking yourself: What does this all have to do with you? Well, you see, my counterpart in this world had been posing as Harry Potter. It is a long and complicated story, but Tim, his real name is Septimus, told me that you had supported him through thick and thin and that I could trust you explicitly.
I am hoping that you will write back. I would really like a friend in this dimension.
Sincerely yours, Septimus Severus Snape
Dear Mr. Snape
First of all, I must tell you that it was quite a surprise to have you write me. I, however am very grateful to have you do so. Could you tell me how Harry, that is Tim, is doing? I have not heard from him all summer and so do not know if he is okay.
Does your being a dimension traveler mean that you are here to say, or only visit? Will Tim go to your dimension?
As for friends, you might want to talk to Ronald Weasley and Hermione Granger, They are Harry's best friends and I am sure they will not hesitate to take you into their fold.
I look forward to hearing from you soon,
Remus Lupin