In an article I published elsewhere on what sets Objective Characters apart from Subjective Characters, I described how Objective Characters represent dramatic functions in a story whereas Subjective Characters represent points of view.
The Protagonist is an example of an Objective Character whose function is to be the prime mover in the effort to achieve the story’s goal. The Main Character is an example of a Subjective Character as it represents the audience position in a story – a point of view.
Authors frequently assign the roles of both Protagonist AND Main Character to the same player in the story, creating the stereotypical “Hero”. (Note that a Hero is a stereotype, not an archetype. The function of a Protagonist is archetypal, but combining that function into the same player as the one assigned as the audience position in the story – the Main Character – is a convention of storytelling, not a necessity of dramatic structure. This makes a Hero a stereotype, rather than an archetype.