Your Brother, Remus Lupin.

"Losing a best friend is like losing air. It's painful and slow and there's nothing you can do about it. You just watch the world disappear."

_
[ The Letter Series.]

Remus Lupin was an only child but he certainly had brothers.
Brothers that stood by his side after knowing who he was and who loved him just as one.
In one night, all of his brothers were gone.
Feeling less like a human at every passing day, he tries to stay in touch with his real self by writing.
Writing letters to his brothers that left him after promising they would stay forever.