doppelganger.

Main Entry: dop·pel·gäng·er
Variant(s): or dop·pel·gang·er /'dä-p&l-"ga[ng]-&r, -"ge[ng]-, "dä-p&l-'/
Function: noun
Etymology: German Doppelgänger, from doppel- double + -gänger goer
1 : a ghostly counterpart of a living person

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.
http://webhome.idirect.com/~donlong/monsters/Html/Doppelga.htm

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