Who is Narsit from Persian Letters. Translated with an Introd. and Notes, by C. J. Betts and what is their importance?

Narsit is a black slave. It is Narsit who informs Usbek that the head eunuch is dead. Narsit, however, fails to open a letter a letter written to the head eunuch by Usbek, as mistake that leads Usbek to declare he is stupid.