Doppelganger!

Meaning "double walker" a doppelganger is a shadow-self that accompanies every human. Only the owner of a doppelganger can see it, otherwise it is invisible to human eyes. Dogs and cats have been known to see doppelgangers. Providing sympathetic company, a doppelganger almost always stands behind a person, and they cast no reflection in a mirror. They are prepared to listen and give advice to humans, either implanting ideas in their heads, or a sort of osmosis. It is said to be bad luck if it is seen, and rarely a doppelganger will make itself visible to friends or family, often causing great confusion. Doppelgangers can be mischievous and malicious.