An advocate of laissez-faire capitalism is a capitalist.
The words capitalism and capitalist are used in two different but related senses: one in a compartmentalized sense within the specialized science of economics (that studies the nature of production in a division of labor society), the second is in a broader sense of politics (the branch of philosophy that examines the fundamental nature of social systems).
Economically, a capitalist is someone who invests capital in a business concern. Within the specialized domain of economics, a person who invests capital in a business concern is recognized as a capitalist, regardless of whether one advocates capitalism politically or not, e.g., Frederique Engels, Warren Buffet, and George Soros are economically capitalists.
Politically, a capitalist is an advocate of laissez-faire capitalism.
More broadly, that is philosophically and politically, only an advocate of capitalism is as a capitalist, e.g., philosopher-novelist Ayn Rand is ideologically a capitalist, factory-owner Frederique Engels (and co-author with Karl Marx of The Communist Manifesto) who advocated communism is politically anti-capitalist. Though economically Engels came from a wealthy background (which he used to subsidize and promote Karl Marx), politically he is recognized as a socialist/communist because of his ideas.
Similarly, billionaires Warren Buffet, Bill Gates, Ted Turner, and George Soros — can be economically compartmentalized as capitalists — but philosophically they are not capitalists as they do not advocate capitalism on principle, but are advocates of “mixed economy” statism (capitalism combined with anti-capitalist elements) to various degrees. Soros being more anti-capitalist than Buffet. Soros, like billionaire Ted Turner, is a “socialist at heart.”
The battle over capitalism is not just an argument over wealth; it is a philosophical one over the individual’s inalienable right to life, liberty, and the pursuit of their happiness.