The Collaborative International Dictionary of English v.0.48:

Protagonist \Pro*tag"o*nist\, n. [Gr. prwtagonisth`s; prw^tos
   first + 'agonisth`s an actor, combatant, fr. 'agw`n a
   contest.]
   One who takes the leading part in a drama; hence, one who
   takes lead in some great scene, enterprise, conflict, or the
   like.
   [1913 Webster]

         Shakespeare, the protagonist on the great of modern
         poetry.                                  --De Quincey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

protagonist
    n 1: a person who backs a politician or a team etc.; "all their
         supporters came out for the game"; "they are friends of the
         library" [syn: supporter, protagonist, champion,
         admirer, booster, friend]
    2: the principal character in a work of fiction [syn:
       protagonist, agonist]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "protagonist":
   Maecenas, abettor, actor, admirer, advocate, aficionado, angel,
   antagonist, antihero, apologist, backer, bit, bit part, buff, cast,
   champion, character, cue, danseur noble, defender, dependence,
   diva, encourager, endorser, exponent, fan, fat part, favorer,
   feature attraction, feeder, first tragedian, friend at court,
   headliner, heavy, heavy lead, hero, heroine, ingenue,
   jeune premier, lead, lead role, leader, leading lady, leading man,
   leading woman, lines, lover, mainstay, maintainer, moving spirit,
   paranymph, part, partisan, patron, person, personage, piece,
   prima ballerina, prima donna, prime mover, principal, promoter,
   reliance, role, second, seconder, sectary, side, sider, singer,
   soubrette, sponsor, stalwart, standard-bearer, standby, star,
   straight part, superstar, support, supporter, supporting character,
   supporting role, sustainer, sympathizer, title role, upholder,
   villain, votary, walk-on, walking part, well-wisher