What was about cloning charllotte? I didn't get that part? What was that for?

Those chapters were to give more background to the characters that didn’t get much focus in the main story. It doesn’t really have anything to do with the main story really because the two were barely involved. Since charlotte was the only successful resurrection done by someone other than the demon, the demon was trying to replicate her to figure out how Will, a mere human was able to do it and in figuring that out it could help him figure out how to reverse it so the other guy could finally die