By definition, the Unknown Citizen has no identity. With the related concept of the Unknown Soldier, it is the soldier’s physical remains, or dead body, that cannot be identified. But for the Unknown Citizen, it is more that his life was so conventional that he did not distinguish himself in any way from his fellow citizens. There must be thousands, even millions, of Unknown Citizens out there, about whom little can be said except that they didn’t get in anyone’s way. On the other hand, you might think that there is nothing wrong with being "unknown," and that the poet is being elitist.
The Unknown Citizen isn’t a particular person – he represents the average of all people in a society.