A lifesaver.
An attendant, usually an expert swimmer, employed to save swimmers in trouble or near drowning at a body of water.
A sturdy metal bracket fixed in front of each of the leading wheels of a train to deflect small objects away from the wheels to prevent derailment.
A bodyguard or unit of bodyguards, a guard of someone's (especially a king's) life or person.
A person who saves someone, rescues another from harm.
A child who is born to provide an organ or cell transplant to a sibling who has an otherwise fatal disease (used in combination, with "sibling", "baby", "child", "brother", "sister", etc.)