the most important character in something
protagonist - hero
A ‘protagonist’ is the central figure in something, particularly a story, film, or play. This formal word can also be used to describe a leading figure or main proponent of a cause, struggle, or idea. Such people are often, but not always, ascribed heroic qualities. It is more commonly used in written English than everyday conversation.
The protagonist in the novel is a down-on-his-luck detective who is trying to solve a mysterious murder.
The main protagonist in the current movement for educational reform is a local councillor who had been a teacher for 30 years.
Jane often supported radical political groups and ideas, and generally faced a lot of criticism for her protagonism on their behalf.