Source : Webster's Revised Unabridged Dictionary (1913)

Resister \Re*sist"er\ (-?r), n.
   One who resists.

Source : WordNet®

resister
     n 1: someone who systematically obstructs some action that others
          want to take [syn: {obstructionist}, {obstructor}, {obstructer},
           {thwarter}]
     2: someone who offers opposition [syn: {adversary}, {antagonist},
         {opponent}, {opposer}] [ant: {agonist}]