Existentialist (noun)

1. A person who subscribes to existentialism.
2. A person who emphasizes the freedom and choice of the individual and the inherent meaninglessness of life.

Origin:

Formed from the noun existentialism + -ist.

Examples:

1. He is an existentialist philosopher.
2. She is an existentialist writer.
3. The novel was written by an existentialist author.
4. He is an existentialist thinker.
5. The existentialist movement is gaining popularity in the recent years.