D.Gray-man, Darker than Black: Kuro no Keiyakusha Recommendations
Both have a guy who seems innocent on the outside, but have special powers and connected to secret organisations which battle against certain phenomena which affect people within the city; all part of many principles tying up to form a large plot. Both have the same dark supernatural power which takes over certain people which they must overcome.
Not that similar as far as plot is concerned. Both have a dark atmosphere. I just think if you like one then chances are you will like the other.
The two have "dark background/theme". Both main characters work mission by mission for a certain organization. Both main characters act as a "bridge" between two conflicting sides (e.g. between human and their opponent on each anime respectively).
both have mc who have trusted partners and dark world, and mc also have a bad memories about their childhood