The Collaborative International Dictionary of English v.0.48:

Justifier \Jus"ti*fi`er\, n.
   One who justifies; one who vindicates, supports, defends, or
   absolves.
   [1913 Webster]

         Justifiers of themselves and hypocrites. --Strype.
   [1913 Webster]

         That he might be just, and the justifier of him which
         believeth in Jesus.                      --Rom. iii.
                                                  26.
   [1913 Webster]

WordNet (r) 3.0 (2006):

justifier
    n 1: a person who argues to defend or justify some policy or
         institution; "an apologist for capital punishment" [syn:
         apologist, vindicator, justifier]