Reformer (noun)

1. A person who advocates or works for political or social reform, especially to improve the condition of the poor or oppressed.
2. A person who advocates or works to improve a particular aspect of society or an institution.

Origin:

From middle english reformer, from old french reformateur, from latin reformat-, past participle of reformare "form again".

Examples:

1. He is a political reformer who is fighting for the rights of the marginalized.
2. She is a leading reformer in the field of education.
3. The reformer was instrumental in the passage of a new law to protect workers rights.
4. The reformer's ideas were ahead of their time, but eventually they were accepted.
5. The group of reformers were trying to bring about change in the political system.