Fetch

The Fetch is a supernatural creature from Scottish myth. They are thought to be made of all the negative qualities and traits of a person. Everyone is believed to have one, but it is suppressed by conscious and societal laws. If someone is disturbed or unbalanced enough, the Fetch can separate from the person and become a whole separate being. Sometimes they remain completely invisible and whisper temptations into the ear of their better half. This may cause one to believe they are insane, or cause them to become insane. They can move objects and harm people and can also manifest looking identical to their better half.

Citations:

Maberry, Jonathan. Vampire Universe: The Dark World of Supernatural Beings That Haunt Us, Hunt Us, and Hunger for Us. United States, Kensington Publishing Corporation, 2006.