General Synonyms:
* Main character
* Lead character
* Hero (implies positive qualities)
* Heroine (for female protagonist)
* Lead
* Central figure
* Principal character
* Chief character
Synonyms emphasizing the protagonist's role in the story:
* Driver of the plot
* Key player
* Central actor
* Focus of the narrative
* Agent of change
Synonyms emphasizing the protagonist's motivations:
* Motivator
* Driving force
* Initiator
* Catalyst
* Agent of action
More nuanced synonyms:
* Antagonist's foil (when the protagonist's actions are driven by the antagonist's actions)
* Anti-hero (a protagonist with morally ambiguous or questionable qualities)
* Tragic hero (a protagonist destined for a downfall)
* Everyman (a protagonist representing the average person)
The best synonym to use will depend on the specific context of your writing.