emissary
A trusted person sent to represent someone on a mission.
An emissary is a person sent on a special mission to deliver a message or represent someone else, usually someone important. When a king needed to communicate with another kingdom in medieval times, he would send an emissary to speak on his behalf. When countries negotiate peace treaties today, they often send emissaries to have preliminary discussions before the presidents or prime ministers meet in person.
That's exactly what an emissary does: they're sent out to accomplish something specific. A school principal might send a student emissary to welcome visitors and explain how the school works. A business leader might send an emissary to meet with potential partners in another city.
What makes someone an emissary rather than just a messenger is the trust and authority they carry. An emissary doesn't just deliver words like a mail carrier does. They represent the person who sent them and often have the power to make decisions or negotiate on that person's behalf. When an emissary speaks, it's almost as if the person who sent them is speaking directly.