Kanchuki is an honest, elderly and reliable person, often a eunuch, held in trust by the king and queen in ancient Hindu world.
Kanchuki was free to move in all parts of the palace, including the harem and royal court. But the movements are restricted to the palace only. The person was extremely duty bound and always at the beck and call of his master or mistress.
In accordance with his position and characteristic, the duties of a kanchuki included carrying of messages to and from the king and the queen and receiving visitors with due decorum.
In view of the unique nature of his functions in normal life, kanchuki was adapted by dramatists.
In Sanskrit dramas the person is a minor character carrying on the functions mentioned above. This would mean that the character of kanchuki would be only seen in higher types of drama involving the kings and queens.
The kanchuki is always soft spoken and precise in expression, and not vivacious like some other characters, such as the vidshaka or jester. However, the presence of kanchuki is essential when occasions arise.
It is also to be noted that, being a learned character, the speech of kanchuki would always be in Sanskrit and not in prakrit, which is used by servants and other menial members of the palace.
Source:
A History of Sanskrit Literature (1993) by Arthur Berriedale Keith New Delhi Motilal Banarsidass
Encyclopedia Of Hinduism - Volume V - page 448 - IHRF