Dear Professor Dumbledore,
Your explanation on how you answer letters despite being dead makes sense. But how about the other dead hero's? Sirius Fred and Cedric to name a few.
Signed,
The ever curious Ravenclaw
There would be a pause as the long dead hero's all stood looking at the author, Dumbledore's portrait, then back to the author.
"Well?"
"We tell them the truth" chimes one, his voice unmistakable like Remus'
"What?! We can't do that!" replies Fred
The author would clutch the quill worriedly, could she really reveal this dramatic secret? Let out the explanation as to how replies came from all of the Harry Potter heroes, alive and dead?
A note, in small, messy handwriting, worse than even Ron Weasley's would scrawl:
'Dear Ever Curious Ravenclaw,
I regret to inform you that before I could give your letter to Dumbledore, it went missing. Luna blames Nargles.
Sorry!
An apologetic author'
There was a look of approval from Dumbledore, a sigh from Remus and a smirk from Fred. It was too big a secret to share after all.