The crime-fighting vigilante The Shadow became one of the most famous pulp heroes of the 20th century and has been depicted in comic books, pulp novels, television, video games, and motion pictures. But The Shadow, who had the power to "cloud men's minds" so they could not see him, is in all of us. And it is this shadow within that often distorts our insight, blinding our virtue. If we've learned anything when it comes to the dark side of self, our most powerful enemy is quite literally our own face in the mirror.
The Shadow, a symbolic character and supporting role that is defined by the hero's shortcomings, is most often brought to life as a split personality (Mr. Hyde to Dr. Jekyll / The Hulk to Bruce Banner) or as a villain (Darth Vader / Lord Voldemort) who shares similar qualities or traits with the hero. Luke Skywalker's largest obstacle is not in defeating the Empire; it's in destroying his father and refusing to embrace the power of the Dark Side.
But because The Shadow is a reflection of the protagonist's flaws, the hero tries to avoid The Shadow so that he/she doesn't have to confront his own faults and face the one thing he fears most: becoming The Shadow himself.
In David Cronenberg's History of Violence, for example, murderous mobster Joey Cusack (Viggo Mortensen) goes so far as to reinvent himself to become a completely different person – small town mild mannered Tom Stall – in an attempt to wash away his past, but no one can erase history. The dark shadows we've left behind always seem to find a way of slithering back. So squeeze your character; force him to face The Shadow, but let your character decide: Friend or Foe?