The Underborns are the females of the synthetic Nephilim. They were created as genetic experiments by Mardon and grown in his lab; thus they have no parents. After they matured and he could observe the effects of each genetic alteration for future reference, most were kept as slaves in the magnetite mines.
As they were experiments, there is a great deal of variation. This is displayed in Enoch's students: every skin color was present, and their hair ranged from black, thick, and curly to baby-fine and frosty white. Sapphira and Acacia were at the pale end of that spectrum, with wispy, pure white hair, unnaturally pale skin and saphire- blue eyes.
Most of the underborns were less intelligent than normal humans. This was deliberate, as Mardon aimed to create slaves who would obey without asking questions and would be unable to form plans of rebellion. However, their slowness was at least in part due to Morgan's influence. In general, those that showed undesirable intelligence where pushed into the magma river. However, instead of dying as Morgan assumed they would, they went through the portal there and studied with Enoch until Sapphira came to release them.