Rebel Soldiers 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. In the Strife: Veteran Edition, the Rebel Soldiers are found to be follow the player anywhere they go with poor AI path finding (this also happens during afore mentioned battle scenes). This can be useful in guiding them to where you may need their help. Not that they'll last very long. (Note although rare, some invisible monster barriers will also be a barrier to them as well)
Note: A constant albeit annoying magazine clip clicking/loading into their guns is actually a sign that they have a target and are attempting to engage and will walk away from the player and open some doors to engage said target.
In The Original Strife: Veteran Edition, the Rebel Soldiers can be summoned with the Teleporter Beacon in Capture the Chalice games to protect your team's Chalice for you. These Soldiers sport new colors / shades to match their teams of blue and red.