I can’t be the only one that thinks this but I’m convinced that Dazai is the main character, not Atsushi.
Sure the story is told from Atsushi’s point of view, but Dazai is usually the one fueling the action. We even get to see Dazai’s past for four episodes in the anime.
The movie poster convinces me of this even more, being that Dazai and Chuuya are pictured front and center.
Even when Dazai’s been MIA(ex: capture by Port Mafia) he’s still been important to what’s happening in the story.
It makes sense that we would follow Atsushi since we seem to be going by the rules of “person being introduced to his new surroundings as the audience is” and it makes it easier to explain things if one character has no idea what’s going on.
I wouldn’t be surprised if he was the Protagonist all along.