I think this is one way to look at it, at least for the books. For the TV show, over all the seasons it seems like three main characters have emerged (based on

I think this is one way to look at it, at least for the books. For the TV show, over all the seasons it seems like three main characters have emerged (based on longevity, screentime, and plot importance): Daenerys, Tyrion, and Jon Snow. You could perhaps argue Cersei as well - iirc she has more POV chapters and page-time in the books than any other character. An alternative view is that the main character(s) change from book to book and season to season. Ned was probably the main character in the first book & season one, Robb replaced him in book/season two, with similar evolutions for other "temporary-mains." Though really the first explanation seems better: a "main character" just doesn't fit the structure of ASOIAF very well.