In the Company of Shadows series description:

Years after the bombs of WWIII have changed the physical and political landscape, the Agency ruthlessly works behind the scenes to take down rebel groups that threaten the current government. Their goals justify all means. Hsin Liu Vega (Sin) is their most efficient and deadly assassin ever. However, he tends to go off on unauthorized killing sprees and somehow his assigned partners all end up dead under suspicious circumstances. That is why the Agency has had him locked up in a box on the fourth floor for years. But now they think it is time to put the psychopath back in the field. With a new partner.

Boyd's mother, a high ranking Agency official, volunteers her teenage son for the position. Boyd is not afraid of death. In fact, his life has been such an endless cycle of apathy and despair for the last few years that he'd welcome it.

Can these two broken men form an efficient partnership? Can they learn to trust anyone, let alone each other? Sin doesn't give a fuck about the Agency, or the androgynous boy who holds the remote to the shock collar that is supposed to control him.

On the other hand, Boyd is strangely unafraid of the man everyone calls 'monster' and Sin seems reluctant to let his keeper get himself killed on any missions. Yet.