Rebels are allies from Strife. They are basically like Acolytes, only fighting on the player's side. They are human rather than part mechanical, and therefore move a lot faster and die a lot easier. They drop the same ammo when killed, and will fire on the player if he attacks Macil before a certain point in the game. It is possible to get caught in their crossfire, though usually they get caught in the player's first, as their AI is not advanced. In the maps utilizing many rebel troopers fighting against The Order, they tend to die like flies.
You can call a group of Rebels to help you on a mission by using a Teleporter Beacon, but they mostly hang around near where the Beacon was placed.