WordNet (r) 3.0 (2006):

soul mate
    n 1: someone for whom you have a deep affinity

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "soul mate":
   Antony and Cleopatra, Pelleas and Melisande, Romeo and Juliet,
   Tristan and Isolde, ally, alter ego, analogon, analogue, associate,
   bill-and-cooers, brother, close copy, close match, cognate,
   companion, complement, congenator, congener, coordinate, correlate,
   correlative, correspondent, counterpart, equivalent, fellow, image,
   kindred spirit, like, likeness, lovebirds, mate, near duplicate,
   obverse, parallel, pendant, picture, reciprocal, second self,
   similitude, simulacrum, sister, soul mates, such, suchlike, tally,
   the like of, the likes of, turtledoves, twin