HERALDS

"I am C-3PO, human-cyborg relations.  This is my counterpart R2-D2"

 - C-3PO, "Star Wars"

The Heralds often fulfill other roles within the story, they may be Helpers or Threshold Guardians.  But in this regard they serve as harbingers and messengers.  They deliver the message of the Call to Adventure.  Think of Gandalf in The Hobbit (yes he is also MENTOR and SUPERNATURAL AID) but at first he is simply a HERALD of an adventure to come.  C-3PO and R2-D2 are almost the perfect HERALDS, both in "Star Wars: A New Hope" and in "Star Wars: The Return of the Jedi".  In both cases, they deliver a message that signals that there is significant change to come in the ORDINARY WORLD;  first with Leia's message asking for help, and secondly with Luke's message warning Jabba to let Han go.  Another good example of the HERALD, and also showing how small this role can sometimes be, 

Also, keep in mind that the HERALD does not necessarily have to be a person.  It can also be a force, such as a storm.  Basically anything that acts as  a harbinger to signal to the hero that a choice has to be made --  that they are at the crossroads and they can heed the Call to Adventure or ignore it.