General terms:
* Advocate: A person who publicly supports or recommends a particular cause or policy.
* Supporter: A person who approves of and helps a cause, person, or activity.
* Proponent: A person who argues in favor of a particular cause or policy.
* Champion: A person who fights for or defends a cause.
More specific terms:
* Spokesperson: A person who speaks on behalf of a group or organization.
* Lobbyist: A person who tries to influence politicians or other officials on a particular issue.
* Activist: A person who works to achieve political or social change.
* Crusader: A person who fights passionately for a cause.
Other terms:
* Cheerleader: A person who enthusiastically supports someone or something.
* Defender: A person who protects someone or something from attack.
The best term to use depends on the specific context. For example, you might call someone an advocate for animal rights, a supporter of a political party, or a lobbyist for a particular industry.