Zone

Zone is a one shot manga written by Katsura Hoshino that introduced the main concepts of D.Gray-Man. Due to the positive reception it recived she began work on the later.

Differences
There are several differences between Zone and D.Gray-Man:
 * Akuma are created by one signing a contract instead of reviving the dead.
 * The main character was not Allen but a similar character named Robin who instead of being cursed by an Akuma was an Akuma herself converted by Cross by having Innocence grafted to her.
 * Lenalee is not member of the Black Order but instead a waitress at a chinese resturant in Japan, and is half Japanese.
 * The Earl hasn't taken control of Japan and it is free from his influence for the most part.