The Collaborative International Dictionary of English v.0.48:

Pursuer \Pur*su"er\, n.
   1. One who pursues or chases; one who follows in haste, with
      a view to overtake.
      [1913 Webster]

   2. (Eccl. & Scots Law) A plaintiff; a prosecutor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pursuer
    n 1: a person who is pursuing and trying to overtake or capture;
         "always before he had been able to outwit his pursuers"
         [syn: pursuer, chaser]
    2: a person who pursues some plan or goal; "a pursuer of truth"

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "pursuer":
   addict, adherent, admirer, adorer, amorist, appendage, attendant,
   buff, bug, cavaliere servente, chaser, collector, courtier,
   dangler, demon, dependent, disciple, eager beaver, energumen,
   enthusiast, faddist, fan, fanatic, fiend, flunky, follower,
   following, freak, great one for, hanger-on, henchman, hobbyist,
   homme de cour, hound, hunter, infatuate, lover, nut, paramour,
   parasite, partisan, public, pursuant, pursuivant, quester,
   rhapsodist, satellite, sectary, seeker, shadow, stooge, successor,
   sucker for, suitor, supporter, tagtail, tail, trainbearer,
   visionary, votary, ward heeler, wooer, zealot



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PURSUER, canon law. The name by which the complainant or plaintiff is known
in the ecclesiastical courts. 3 Eng. Eccl. R. 350.