The Preacher and The Stranger from "High Plains Drifter" are the same character.
In "High Plains Drifter", it's highly implied that the Stranger is the ghost of Marshall Duncan returned to punish the town of Lago and avenge himself upon his murderers. After having accomplished said vengeance, perhaps either of his own accord or as an edict from Heaven, the Stranger/Duncan is to wander the earth and help those in need. The Preacher's garb and priestly-attitude may be a sign of Duncan's more righteous conversion.- The movies do set up two sides of the same coin. This would be true if the speculation above is true, or if the Stranger and the Preacher are different entities. Both are avengers of past wrongs. There are implications in "Pale Rider" that Preacher is there in a similar role, but he has a "Better attitude."
