From Halopedia, the Halo wiki
Protector is a Halo: Reach, Halo 4 and Halo 5: Guardians multiplayer, campaign, Firefight and Spartan Ops medal. It is awarded to a player who saves a teammate from death by killing the teammate's antagonist.
The Bungie weekly update in which it was revealed describes the mechanics of the medal as follows:
- " In "Protector"’s case, after a certain amount of damage has been done to a Player A by Player B, if Player C kills Player B, he earns the Protector Medal. Player A receives a message (in the same place as messages like "You sniped JimmyAwful") that says Player C saved your life. "
In Halo: Reach it is represented by a life ring with a red and white yin-yang symbol in the center on a four-point star. In Halo 4 it is represented by a white 'x' on a red road sign with a halo and wings, imposed on a blue circle and grey cross. In Halo 5: Guardians it is represented by a white 'x' on a red road sign with a white halo and wings, on top of a gray pentagon.