Every character has his or her "role" to play in a story, just like each actor or actress has a role to play in a drama. A role is something's purpose, meaning and function. Even though characters in a story may seem real (or unreal), they are still a narrative element, like plot or setting. Like all narrative elements, characters are ingredients that make the story what it is. If a story were like a painting, and all the narrative elements were brushstrokes, consider the main character one of the most prominent strokes.
Story Of A Girl
The most basic way to analyze a main character's role is to look at how she affects the plot - the events that unfold in a story. How she affects the plot depends on her personality and her situation. For example, Lyra Belacqua in Philip Pullman's "The Golden Compass" is a bold, fearless, adventure-seeking character, whose risk-taking personality frequently puts her in danger, thus advancing the plot and heightening the suspense. That she would leave home in the first place is believable because she is an orphan (or so it appears). Plotting the major events of a story and then determining the main character's influence on each event helps illuminate how the protagonist serves (or doesn't serve) as a catalyst for plot development.
Two Sides To Every Story
Main characters usually influence the characters around them, too. In Khaled Housseini's "The Kite Runner," Hassan's betrayal of Amir directly contributes to Amir's tragic life, resulting in Hassan's lifelong struggle with guilt. Main characters can harm, encourage, teach, frustrate or change the antagonist or another minor character. Layered, dynamic interactions between characters in a story demonstrate the depth and quality of their relationship. Mapping the characters' personalities and relationships to each on other on a piece of paper can help one visualize the role of the character concerning others in the story.
That's My Story
A sometimes overlooked role of a main character is his effect on the reader. Part of the main character's role is to get the reader invested in the book or short story. If the reader doesn't care about the main character and what will happen to him, she won't care about the book. A narrator elicits interest in the protagonist through several means, including empathy and self-identification. Because the little match girl is poor and downtrodden, the reader pities her and begins to hope for her future. Many readers can identify with a hesitant, fearful protagonist like Frodo Baggins and feel as though it is really they themselves on the adventure. They want to finish to the novel to see how the character they identify with surmounts challenges. In the middle of a story, consider how important the main character is and how the reader should feel about them.
End Of Story
While making a distinction between round (realistic) and flat (archetypal) characters is important, it's also essential to note that all characters are equally inventions of the author and all have a role to play. Some of them may present an ideal to the reader, such as brave, virtuous Aragorn from “The Lord of the Rings,” or some may illustrate a moral to a reader, such as Elizabeth and Mr. Darcy overcoming their prejudice and pride, respectively. This depends partly on the author's purpose in creating the main character, whether to teach, satirize or draw admiration for. Upon finishing a book, reflect on the main character's larger, more general importance in the novel and why the author didn't create the character differently (or what would have happened if she did).