I kind of want to make the distinction between main character of the game (Vaan) and main character of the story (Ashe). If you want to speak in drama terms, I think this quote from Wikipedia is pretty relevant:
The terms protagonist, main character and hero are variously defined and, depending on the source, may denote different concepts. In fiction, the story of the protagonist may be told from the perspective of a different character (who may also, but not necessarily, be the narrator).
Ashe would be the protagonist, while you explore the story from Vaan's perspective.