In the pantheon of archetypal figures who appear and re-appear regularly in fiction of all kinds, there is another kind of antagonist. This is the character construct who bears a close resemblance to the protagonist himself or herself. This character is usually working for the main antagonist, or shares the goals of that figure, so he or she is both an enemy to the protagonist and yet very much like the protagonist in many ways.
This is the Shadow Protagonist.
• In The Lord of the Rings, this is Gollum, the ancient hobbit twisted by the One Ring.
• In the larger Star Wars series, Darth Vader is the Shadow Protagonist while the Emperor is the main antagonist.
• In the Harry Potter series, Malfoy is the Shadow Protagonist to Harry.
In Captain America: The Winter Soldier, Steve Rogers’ Shadow Protagonist is clearly the Winter Soldier himself, his former partner transformed into a brainwashed assassin.
Other examples abound: Orlick in Great Expectations, Rochester’s first wife in Jane Eyre, M’lady in The Three Musketeers, and so on. These figures act as a kind of ‘deputy antagonist’ in many cases.
In the case of most of these constructs, it’s possible to see in the story how the protagonist might have walked down a different path and become the shadow protagonist:
• Frodo (or Bilbo, in The Hobbit) could have been Gollum, and could become more like him through overuse of the Ring of Power.
• Luke Skywalker could become like Darth Vader and is explicitly tempted to do so.
• Harry Potter almost ends up in the House of Slytherin and is throughout the stories seduced to become like Malfoy .
• Steve Rogers is haunted by the Winter Soldier - ironically, it is the Winter Soldier who becomes Captain America briefly in the comics.
The primary purpose of the shadow protagonist, like that of the antagonist, is to highlight the desperate needs of the protagonist. Such is the burden of the inner character vacuum of the protagonist that he or she could be twisted into becoming just like their shadow.
I’m sure you can think of other examples yourself.
The Shadow Protagonist is yet another common tool used by writers to attract and engage readers or viewers.