I've spoken previously about Dramatica and the use of the Impact Character to better define the Protagonist's through line, and round off the main story is a more dramatic fashion, providing us with a character arc, and character development. My screenplay tutor provided the following link:
And also the Youtube clip:
... to prove the point about Impact Characters and their place in story.
How cool is that?
Anyhoo, to me this raises the point of all these Main and Impact Characters play off each other, and only one of them gets to change... but what about if they do both change? If they both try to better themselves and break from the mold - a third character gives the lines, of how similar they are. Could that work? I don't see why not.
That said, Peter Parker changes in Spidey 3, and so does his Impact Character - Harry Osbourne - but of that I'll say no more.
And there's more:
Further then, to that, the following analysis of Training Day makes the observation of having a third character advise that Main and Impact are alike: