Abraham Lincoln, is one of the five Historic Heroes, he boosts the number of people that you gain from population upgrades.
Given the heavy dependence on people for completing missions and increasing rank, this is probably the most valuable hero available.
Abraham Lincoln's effect multiplies with the effect of other heroes. This means that if, for example, you have a level 5 Eleanor of Aquitaine (x32 people from Knowledge upgrades) and a level one Abraham Lincoln (x2 people from all upgrades), then every upgrade from the Medieval Age will grant you x64 more people.