Good Twin, Evil Twin.

doppelgänger (pronunciation (help·info)) is the ghostly — in some cases, the physical — double of a living person. The word "doppelgänger" is a loanword from German, in which language it is written (as with any German noun) with an initial capital letter: Doppelgänger. The word derives from Doppel ("double") and Gänger ("goer").

Now... what happens when your twin is EVIL? check it by clicking on the picture!