18+ Tokumei Kakarichou Tadano Hitoshi

Hitoshi Tadano is an assistant manager of an advertisement company who seems to be nothing but a dull and incapable "nobody." While most of the employees wonder why such a passive and incompetent person like Hitoshi is yet to be laid off, there is just one person who heavily relies on him as a "savior" to the company, the CEO. In fact, the "Mr. Nobody" is simply a disguise from the real Hitoshi, as he is under various special missions of the CEO to quietly carry on his "extraordinary" task to fight against the corruption of the company.

(Source: AsianWiki)

Included one-shot:
Volume 9: Sanjuu ni Nattemo Doutei Monogatari