In the book "The Writer's Journey", author Christopher Vogler, the shape shifter signifies the anima or animus archetype. Besides representing the repressed parts of the opposite sex in self, the shape shifter also represents change. The literary hero who can embrace this archetype can draw on the strength of his hidden self as well by acknowledging the hidden energies the archetype represents. The shape shifter can be a positive or negative character, and sometimes other archetypal characters like the mentor or even the hero takes on traits of the shape shifter from time to time.
Some basic shape shifter stories exist. These include the character that's estranged from his family for being different; the one that's alienated from society; the beast within like Dr. Jekyll/ Mr. Hyde; some aliens stories; romantic comedies; some stories dealing with eroticism and bodily changes and narratives dealing with societal changes and the difficulties they cause people. Although the stories often deal with the supernatural shape shifters, they don't always have to be.
The femme fatale represents a common non-fantasy shape shifter type, although it's not limited to that. This character type goes all the way back to Eve and Delilah in the Bible, and includes modern versions like Sharon Stone's character in "Basic Instinct" and Kathleen Turner's character in "Body Heat." Some modern male versions include Michael Douglas' character in "Romancing the Stone" and Captain Jack Sparrow in the "Pirates of the Caribbean" films. "Writer's Journey" author Vogler explains that if a story's hero is constantly asking himself questions like "Is he faithful to me? Is he going to betray me? Does he truly love me? Is he an ally or an enemy?" then aspects of this archetype are at play.
By its very nature, the shape shifter in stories is quite flexible. Although often the shape shifter lands on the literary stage in stories dealing with male/ female relationships, the larger function of the archetype in stories helps the characters within the story to adapt to the ever-changing situation in the story.