extremist

Example:She is a radical who fights for women's rights.

Definition:favoring complete or extreme political or social reform; fundamentally different from established beliefs or practices.

Example:He is a fanatic about his atheism and criticizes religious communities harshly.

Definition:a person who is obsessive, enthusiastic, or overzealous in a political, religious, or other cause or pursuit; someone who is inclined to go to extremes in matters of belief, practice, or opinion.