Darker than Black: Kuro no Keiyakusha, Code:Breaker Recommendations
+Both main characters are similar, both are mysterious and are assassin.
+Both are Action, Mystery, Sci-Fi, Super Power genre.
+Both anime got secret organization or Crime syndicate.
+Both anime main female character got a girl beside them.
Both anime Have bad ass main characters who are assassins with strong powers whom work for a secret organization. In DTB, Hei has an electric type ability, and in Code Breaker Oogami Rei has a flame type ability.
Both involve humans with super-human-powers, not just the main characters, but most of the supporting characters as well. In DTB they are called contractors because they have to make a "payment" of sorts EX: smoke a cigarette, or sing, or fall asleep, etc. after using their ability.
In Code Breaker they don't have payments, but they do have a limit of how much they can use their power
The way you'll never get any explanation on superpowers while the story and it's characters are interesting enough to keep you watching the series through the end.