The Collaborative International Dictionary of English v.0.48:

Sympathizer \Sym"pa*thi`zer\, n.
   One who sympathizes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sympathizer
    n 1: commiserates with someone who has had misfortune [syn:
         sympathizer, sympathiser, comforter]
    2: someone who shares your feelings or opinions and hopes that
       you will be successful [syn: sympathizer, sympathiser,
       well-wisher]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "sympathizer":
   Maecenas, abettor, accessory, accomplice, acquaintance, admirer,
   advocate, aficionado, ally, alter ego, angel, apologist, approver,
   backer, best friend, bosom friend, brother, buff,
   casual acquaintance, champion, close acquaintance, close friend,
   collaborator, condoner, confidant, confidante, conspirator,
   defender, dependence, encourager, endorser, exponent, familiar,
   fan, favorer, fellow, fellow creature, fellowman, friend,
   friend at court, inseparable friend, intimate, lover, mainstay,
   maintainer, neighbor, other self, paranymph, partisan, patron,
   pickup, promoter, protagonist, reliance, repository, second,
   seconder, sectary, sider, sponsor, stalwart, standby, support,
   supporter, sustainer, upholder, votary, well-wisher