Ouroboros

Ryuzaki Ikuo and Danno Tatsuya are orphans who grew up in an orphanage called Mahoroba no Ie. When they were elementary school students, an orphanage staff, who took care of them like an older sister, was killed by someone before their very eyes. The two of them gave statements to the police, but their words were squashed by a police officer with a gold watch and the case was also covered up for some reason. 15 years later, Ikuo has become a detective. Skilled at catching a whiff of cases, he has the highest rate of arrests in his police precinct. On the other hand, Tatsuya has risen to the top of the underworld as a leading member of an organized crime group. A police and a gangster with only each other to trust. The objectives of these two people, who should never have any association, are to locate the gold watchman within the police, find out the truth and expose the darkness of this all-powerful organization.
~~ Adapted from the manga series "Ouroboros" by Kanzaki Yuuya.