@M "Series Regular" contracts automatically make the characters of the actors who are given said contracts "main characters", even if they were to essentially serve as "support" for SMG's Commander Burnham like you're suggesting, because they are going to be the characters with whom she interacts most regularly, even if she were to be interacting with a different 'set' of characters week-to-week. Even if Michael Burnham were to be the sole "POV character" for the series in the way that you're suggesting, she would not be the only "main character", because that is not how television works.