Whom did Karl Marx say would be the ruler in a communist government?

There would be no ruler in a "communist government" to use the terms of this question. This is because under true communism there would be no government at all. It would have withered away and disappeared as being unnecessary. The various industries in the economy would be run by administrators for the benefit of the entire society rather than individual profit. The "proletariat" or working class would not be the rulers, because communism would be a classless society. The terms "proletariat" and "bourgeoisie" refer to social classes and even these would have been wiped out under communism.

Prior to the true communist society and immediately after the overthrow of capitalism, society would go through a period of socialism, which would be ruled by a democratically elected "Dictatorship of the proletariat."
The proletariat, that is the workers who own nothing but the right to sell their own labor, would be rulers after the overthrow of the capitalist sytem, but NOT in a communist government.

There would be no ruler in a "communist government" to use the terms of this question. This is because under true communism there would be no government at all. It would have withered away and disappeared as being unnecessary. The various industries in the economy would be run by administrators for the benefit of the entire society rather than individual profit. The "proletariat" or working class would not be the rulers, because communism would be a classless society. The terms "proletariat" and "bourgeoisie" refer to social classes and even these would have been wiped out under communism.

Prior to the true communist society and immediately after the overthrow of capitalism, society would go through a period of socialism, which would be ruled by a democratically elected "dictatorship of the proletariat."