Defender (noun)

1. A person or thing that defends or protects something.
2. A person who argues in support of a particular cause or proposal.

Origin:

Late middle english: from old french defendeor, from defendre (see defend).

Examples:

1. The castle's walls served as a defender against enemy attacks.
2. She is a passionate defender of human rights.
3. The team's goalkeeper is their main defender.
4. He is a defender of the theory that life on Mars once existed.
5. The company hired a legal defender to represent them in court.