 If you liked
Darker than Black: Kuro no Keiyakusha
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 ...then you might like
Zetman
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Both series contain a dark and gloom atmosphere. While watching the series, there is also a mystery background-like feeling that fills the void of the story.
Both series contain action, violence, and drama. Additionally, both the main protagonists are cold not wanting others to get involved with them.
Both series also contain supernatural elements set in a dark city with an almost dystopian-like background.