Every story needs tension, and one mechanism for supplying that tension is a conflict with an antagonist.
Conflict here doesn’t necessarily mean a physical confrontation, but rather another character whose goals are not compatible with those of the protagonist.
The etymology of the words is interesting. A casual reading suggests the terms come from “pro” and “anti”, that is for and against, but in fact protagonist is from Greek words meaning “first actor”. Antagonist comes from “anti” as one would expect, but in dramatic terms an antagonist is defined by their opposition to the protagonist.
The nature of that opposition can change over the course of a story, Let’s say the MC likes to collect little frog figures. The antagonist might be another collector who wants the same rare figure, the one with the golden stripes and the foil-backed jewel eyes.
This antagonism could lead in all sorts of directions. The antagonist might want the frog as a way to attack the protagonist. Or the antagonist might want all the frogs and the antagonist is just coincidentally looking for the same thing. Or the antagonist might just collect frogs as a side business, and really they’re an evil mastermind who wants to take over the world!!!
(somehow, that last statement required three exclamation marks)
Introducing an antagonist whose goals are larger than the protagonist’s initial interest is a good way to grow a hero. Let’s say that our protagonist learns of the antagonist’s world-domination plans after losing the rare frog, and that the antagonist must be thwarted. At some point the antagonist should learn of the earlier interest in frog figures, and offer the protagonist the rare frog as a way of buying them off.
Earlier in the story this gesture might have worked, but now the protagonist has grown and sees that stopping the antagonist is more important than obtaining the toy frog.
This is obviously a silly example, but this idea of the protagonist uncovering deeper and darker truths about the conflict he or she is part of is quite common. If nothing else, a story where the protagonist knows everything is not necessarily going to be an interesting story.
An antagonist doesn’t need to be a character in the story per se: the protagonist might have the simple goal of getting home, but the weather can be the antagonist here, constantly upending the protagonist’s plans.
For my own writing, I have been working on making my antagonists more well-rounded, considering them to have their own story arcs which my writing offers small slices of. So in Bluehammer, for example, the priests who are seeking power have back-stories just as the protagonists do, and just as other recurring characters do.
Which of course is what I am trying to do with Song at the moment, too, and which I should get back to now.