Source : Webster's Revised Unabridged Dictionary (1913)

Justifier \Jus"ti*fi`er\, n.
   One who justifies; one who vindicates, supports, defends, or
   absolves.

         Justifiers of themselves and hypocrites. --Strype.

         That he might be just, and the justifier of him which
         believeth in Jesus.                      --Rom. iii.
                                                  26.

Source : WordNet®

justifier
     n : a person who argues to defend or justify some policy or
         institution; "an apologist for capital punishment" [syn:
         {apologist}, {vindicator}]