When looking at the Main Character’s Throughline, all other characters are unimportant and should not be considered. Only the Main Character’s personal identity or essential nature is meaningful from this point of view. What qualities of the Main Character are so much a part of him that they would not change even if he were plopped down in another story? For example, Hamlet’s brooding nature and his tendency to over-think things would remain consistent and recognizable if he were to show up in a different story. Laura Wingfield, in The Glass Menagerie by Tennessee Williams, would carry with her a world of rationalizations and a crippling propensity to dream if we were to see her appear again. These are the kinds of things to pay attention to in looking at the Main Character Throughline.
From the Dramatica Theory Book