 If you liked
D.Gray-man
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 ...then you might like
AR∀GO: London Shikei Tokushu Hanzai Sousakan
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The art is a bit similar, Arago and Allen both have "strange" arms, and there's this "don't hurt my friends or you'll pay" type of girls (aka Rio and Lenalee). Also, the police are like the black order, and there's this guy who dislikes the main character.
Also, they deal with supernatural foes and have tragic pasts. and the main charas have something "dark" within them.