This is amazing - a relationship that makes so much sense in your hands, and so beautifully written that everything is deeply felt by the reader, that one has to wonder why it hadn't been thought of before.
Both Moody and Lupin are complex characters, people it is easy to dislike, even as you feel a great deal of sympathy for them. They each wear masks, though of opposite character, to the world, and the silences between them are more eloquent than any long speech could possibly be. You allow us and them to glimpse beneath those masks and make the relationship which grows between them feel as natural and inevitable as the pain which drives it.
The way the relationship ended felt so very real, and that it doesn't make Lupin begin spiraling out of control speaks to how healing it was for him. It left him in better place than he was when it began, and that's about the best one can ask and he understands that.
The end was so poignant and dovetails so beautifully with the original - a perfect fit. Superbly done, Mystery Author!