"Acting on some inscrutable impulse, a parallel version of yourself travels through space and time to destroy you."
This mental defect costs -3 mutation points in character creation.
This is one of the better defects in the game as its effects can be very minimal for a chimera mutant due to the fact that the Evil Twin only appears in new locations you haven't been yet, is only equiped with the stuff you are carrying.
This means you can unequip your weapons and armor each time you go to a new area to spawn the evil twin with no equipment. Making it a very easy fight. Keep in mind that it still does have your mutations, which can still pose a great threat to esper mutants.