The answer might depend on how you’re defining “main character”.
As @mlucas said, Dramatica differentiates between “main character” and “protagonist” (they can be the same, but not necessarily [CORRECTION - they can be in the same “player”, but the roles are not the same]). The separation of the “main character” role (the “I” perspective) from that of the “protagonist” is one of the first insights that convinced me that Dramatica was onto something that I had not encountered in other story paradigms. So that’s one answer.
But you might be asking something related but slightly different, namely, in a novel that has multiple points of view, how do you know which POV is the “main character”? We had a discussion about that here, so maybe that will be useful.