 If you liked
D.Gray-man
|
 ...then you might like
Zone
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Zone is essentially a pilot manga for D.Gray Man. While it differs in many aspects, all the important parts are there. Character backgrounds and interactions are somewhat different compared to D.Gray Man, but their personalities are very consistent between the two manga. There are differences in plot and worldbuilding which means that Zone is not actually a D.Gray Man prequel, and so it can be read at any point before, during or after reading D.Gray man.