@xeno77 That's not in any way true. The protagonist is always a main character, but every main character is not the protagonist. The protagonist drives the story, they are the central focus of the narrative. Stories can very easily be told from the outside, as in from the perspective of someone who isn't actually the protagonist.
Take Sherlock Holmes, for instance. The protagonist of any Sherlock Holmes story is obviously going to be Sherlock himself, but Watson is the POV character in all but three or four.