Kei x Yaku: Abunai Aibou

Ichirou, an investigator at the Tokyo Metropolitan Police Department Public Security Bureau, under the order of his superior, approaches Shirou, a yakuza who is engaging in sexual companionships with politicians, in order to investigate and monitor him. However, he unexpectedly finds out that Shirou is the younger brother of his senior investigator. Ichirou, who always wanted to find his senior who disappeared under mysterious circumstances, secretly teams up with Shirou, and the two of them decide to solve the mystery of his senior's case. While they pretend to be lovers in order to deceive Ichirou's boss...

(Source: Kodansha, translated)