Ouroboros

Two boys, Ikuo Ryuuzaki and Danno Tatsuya, were raised by a mentor they referred to as "sensei." After their mentor is violently killed, they promise to pursue those responsible, including the police who failed to address the case adequately. Fifteen years later, Ryuuzaki works as a detective while Tatsuya leads a criminal organization. Despite their opposing roles, they secretly assist each other in advancing within their careers, aiming to reach positions of power that will allow them to carry out their revenge.