Undead Fighters are a type of enemy encountered in the Silver Land at the Ruins of the Silver Knights. The manner it walks, runs, stands, and attacks mimics Jin's Fighter's stances. It is a undead Silver Knight, because Silver Knights are fighters.
The Fighters were originally slaughtered by the Black Fairy Queen, only to have their bones reanimated by her. They were used to track and eliminate Jin.
Two of them spawn as guardians for the Black Fairy Queen, and will "revive" them if they die.