First Citizen was a title which existed in the parallel to the Roman Empire on the planet 892-IV.
One holder of this title was Captain R.M. Merik. Merik had arrived with his ship, the SS Beagle on this planet in 2262. While many of his crew were subsequently put to death in the arena, Merik, now called Merikus, became First Citizen and also Lord of the Games. (TOS: "Bread and Circuses")
The title of First Citizen is roughly equivalent to the historic Roman Empire's title of "Princeps". This title was held by the ancient rulers of the Roman Empire on Earth. Merik's duties in this role are unknown, since he clearly held this role in a puppet status to Proconsul Claudius Marcus.