Kei X Yaku Bound By Law vol 4 back

Ichiro, an agent with the Tokyo Metropolitan Public Security Bureau, receives orders to make contact with and closely surveil Shiro, a yakuza hotshot whose bed partners include a considerable amount of influential male politicians. The mission takes an unexpected turn when they realize they've both been pursuing the same cold case: the disappearance of Rion Nakaba, Ichiro's senior and Shiro's sister. Determined to uncover the truth, the two secretly team up under the guise of lovers to try and pick up her trail.