The Forever Knight Ninja is a member of the Forever Knights and a member of the Negative 10. He is defeated by Upchuck, who spits his own shurikens back at him. It is left ambiguous as to whether he is a robot, or if there is someone inside the suit. He first appears in "Ben 10 vs. The Negative 10" and does not have a voice actor (being completely mute).
The Forever Knight Ninjas first appearance in the Ben 10 Series.

In "Forever Defense", there are fighters that look suspiciously like the Forever Ninja, possibly alluding that there are more than one Ninja and that being a Forever Knight Ninja is a rank among the knights.