noun (n)

• an opponent who resists with force or resolution(noun.person)

  Example:
  Obstinate withstanders of innovation.
  

  source: wordnet30
• a fighter who holds out against attack(noun.person)

  Synonym:
  defender
  

  source: wordnet30
• One who withstands, or opposes; an opponent; a resisting power.(noun)

  source: webster1913