Kei X Yaku: Bound By Law Vol 15

From the Kodansha edition

Opposite sides of the law flirt with danger--and each other--in a tangled web of political intrigue that is fraught with deception and secrets. A suspenseful mystery sure to get the heart racing in more ways than one, perfect for fans of No. 6 and Banana Fish! 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...

Source: Penguin Random House API