What type of narrative device is the 'evil twin' considered?

The 'evil twin' is recognized as a narrative device that illustrates contrasting qualities primarily because it embodies the duality of human nature or character traits. In storytelling, the concept of an 'evil twin' allows for a direct comparison between two sides of a character, often highlighting themes such as good versus evil, morality, and the struggle within oneself. This contrast can enhance the understanding of the protagonist's qualities, motivations, and growth, as the character must confront or reconcile with their darker side. This device is often used to create tension and complexity in the narrative, allowing readers to explore deeper psychological themes and the consequences of choices made by the characters.