lifesaver
Definitions
A person or thing that saves someone from danger, death, or a difficult situation.
A buoyant ring used to keep a person afloat in the water.
Examples
That extra battery pack was a total lifesaver during our long hike.
Having a friend who can help with childcare on short notice is a real lifesaver.
Synonyms
savior rescuer guardian angel benefactor help in time of need