noun
existentialist
A person who adheres to existentialism, a philosophical movement emphasizing individual existence, freedom, and choice.
彼は実存主義者として、自由と責任についてよく語る。
As an existentialist, he often talks about freedom and responsibility.
実存主義 is the philosophy of existentialism, while 実存主義者 refers to a person who follows it.
Compound of 実存 (existence) + 主義 (doctrine, -ism) + 者 (person). The term is a calque from Western philosophy, likely from German 'Existentialist' or French 'existentialiste'.