on Helping Writers become Authors:
Most of the time when we think about a story’s antagonist, we simply think of the “bad guy.” The antagonist is the character (or force) that opposes the protagonist’s forward progression in pursuit of the main plot goal.
This is fundamental plot theory, linked to the old saw about “no conflict, no plot.”
The antagonist creates that conflict by presenting obstacles to the protagonist’s easy forward momentum. When the protagonist finally overcomes those obstacles, the conflict ends and so does the plot.
But the antagonist is not merely a static force of opposition