The purpose that a character serves within a work is the best and simplest definition of functional significance as it applies to characters in a literary work. When constructing a story, an author uses characters to drive the action. Each character's actions and interactions help define his or her function within the story.
Character function may not strictly be contained within the overall story. Sometimes small details about a character such as a character's name or the frequently occurring color in relation to a character may be a significant clue to the character's function. Scholars have divided characters into several different categories and sub-categories of character function. The primary functions are that of protagonist and antagonist. The purpose of a protagonist is to provide an axis to the story as well as to be a vehicle for guiding readers. In essence, a story is the protagonist's tale. An antagonist's primary role within the story is to act as resistance to the protagonist. Though the two terms are often reduced to "good guy" and "bad guy," this is not always necessarily true. Anti-heroes are sometimes protagonists, as is the case for Satan in Milton's "Paradise Lost." Likewise, antagonists are not always good, likeable or even trustworthy.