None of these archetypes have to be one person each. They may be a person who acts in several of these capacities. The Mentor might also be a Herald.
A Herald issues challenges and announces the coming of important changes. Think of the Heralds in medieval times. They knew the lineages and coats of arms. They identified people, relationships in battle, tournaments and, perhaps, weddings.
In a story, the Hero has succeeded up to this point. Suddenly, he faces a challenge that changes his way of life. His life will never be the same again. He has been called to an adventure, oftentimes by the guise of a Herald. It can be a dream, a person or a new idea. In Field of Dreams a voice tells the Hero "If you build it, they will come." He faced a change.
Our story must face a change also. The Herald gives the character motivation, a challenge and starts the real story. In Earthquake, earth tremors act as Herald. A stock market crash may hold the name of Herald. In High Noon, the Herald is a clerk with a telegraph message. A map and a telephone call herald change to a character in Romancing the Stone.
Heralds represent positive, negative or neutral people. Darth Vader causes the audience to realize something is not quite right.
A Herald may appear through a Mentor, Trickster, or Threshold Guardian character. The Mentor is usually a positive for the Hero. The Trickster and Threshold Guardian act as neutrals.
Usually the Herald appears in Act One although he may appear at any time in the story. However, he is a necessity in every story.
If your story is in progress, try to determine who plays the part of each archetype we've discussed thus far.
5 days ago