A protagonist is the main character in a story, and is often the hero or heroine. The word protagonist comes from the Ancient Greek word prōtagōnistḗs, which means "one who plays the first part, chief actor". 

Here are some characteristics of a protagonist:

Central to the plot: The protagonist is often central to the story's conflict or plot. 

Emotional heart: The protagonist is usually the emotional heart of the story. 

Changing: The protagonist usually makes choices, takes actions, and is changed by the repercussions. 

Opposed by an antagonist: The protagonist is opposed by an antagonist, who creates conflicts and obstacles that test the protagonist. 

Fate closely followed by the audience: The audience or reader most closely follows the protagonist's fate. 

A story can have more than one protagonist, and each subplot may have its own protagonist.