An envoy is someone sent out as a messenger to represent another person or group, such as a country or business.
The saying "Don't kill the messenger" could easily be "Don't kill the envoy," because an envoy is a messenger. The U.S. government has many envoys, as do other countries and many organizations and businesses. In government, an envoy has much less power than an ambassador. The word envoy is also used to describe a brief postscript in poetry or prose that explains or ties up the previous piece before “sending it to readers,” so that’s messenger-like too.