Ushiro no Kedamono

From Blissful Sin:

Fashion designer Munekazu called for a helper, but at first glance the one who answers looks like a professional hit man, with a daring manner; the wild man named Kaoru. He’s reliable and capable, but the super easygoing Kaoru keeps flaunting himself towards Munekazu. Furthermore, the self-confidence locked inside Munekazu’s heart is drawn forth. But then Kaoru says, “Show me a more meek, cute face,” and suddenly kisses Munekazu, stripping him naked—?!