agent in Latin

One who exerts power, or has the power to act.

One who acts for, or in the place of, another (the principal), by that person's authority; someone entrusted to act on behalf of or in behalf of another, such as to transact business for them.

Someone who works for an intelligence agency: whether an officer or employee thereof or anyone else who agrees to help their efforts (for ideology, for money, as blackmailee, or otherwise).