Jamieson, Fausset and Brown on Esther 4:5

5. Then called Esther for Hatach, one of the king's chamberlains, whom he had appointed to attend upon her--Communication with the women in the harem is very difficult to be obtained, and only through the medium of the keepers. The chief eunuch receives the message from the lips of the queen, conveys it to some inferior office of the seraglio. When the commission is executed, the subaltern communicates it to the superintendent, by whom it is delivered to the queen. This chief eunuch, usually an old man who has recommended himself by a long course of faithful service, is always appointed by the king; but it is his interest, as well as his duty, to ingratiate himself with the queen also. Accordingly, we find Hatach rendering himself very serviceable in carrying on those private communications with Mordecai who was thereby enabled to enlist Esther's powerful influence.