Characters Who Must Fight Their Mirrored Selves

In my quest of learning more about character, I'd found a tidbit about heroes and supporting characters that surprised me.  This came from a newsletter written by Chris Soth who created the mini-movie method.  I tend to use it rather than the three act structure.  You can check out his site at www.milliondollarscreenwriting.com. With any hero there's a character arc.  Whether he has to learn to be less selfish, or learn to believe in herself, I find that I connect with the hero when he or she must go through something and come out a better person.  Isn't that our jobs?  To connect the reader to our hero?  And when they don't the story is then called a tragedy.

There are supporting characters that are necessary.  Heroes tend to have a nemesis, possible love interest, straight man or woman, a sidekick, gap-inducing character, etc.  Chris suggests having at least two mirror characters.  One character that shows what the hero can and should become--giving, self-confidence--and another, maybe the bad guy, who shows the reader what would happen if the story was a tragedy.

I understand the purpose of mirrored characters, but I'd never heard of this before.  Have you?