Legal Professionals:
* Lawyer (general term for someone qualified to practice law)
* Attorney (another general term for someone qualified to practice law)
* Counsel (a lawyer retained to give advice or represent a client)
* Advocate (a person who publicly supports or recommends a particular cause or policy)
* Barrister (a lawyer who has the right to plead in the higher courts of law)
Other:
* Representative (someone who acts on behalf of another)
* Agent (someone who acts on behalf of another in business matters)
* Negotiator (someone who tries to reach an agreement between parties)
* Canvasser (someone who goes from door to door seeking customers or support)
* Petitioner (someone who formally requests something, especially in a legal context)
The best synonym will depend on the specific situation and the nuance you want to convey.