Activist (noun)

1. A person who campaigns for some kind of social change.
2. A person who is actively involved in politics and social issues.

Origin:

Activism + -ist.

Examples:

1. He is an environmental activist.
2. She is a civil rights activist.
3. The activists gathered in the city center to protest.
4. The activist group organized a march to demand change.
5. The company faced backlash from activists for their lack of sustainability practices.