A spokesperson or mouthpiece; someone who speaks on behalf of others and expresses their opinions or feelings.
その議員は弱者の代弁者として知られている。
That legislator is known as a spokesperson for the disadvantaged.
弁護士は依頼人の代弁者として法廷で主張した。
The lawyer argued in court as the client's representative.
市民の声を代弁する団体が設立された。
An organization was established to speak on behalf of citizens.