Who is Nyai Ontosoroh from This Earth of Mankind and what is their importance?

Nyai Ontosoroh is also referred to as Mama by some of the characters. She is the concubine of Herman Mellema and mother of Robert and Annelies. She becomes a second mother to Minke, as well. She leads the family business in the absence of Herman Mellema, who had an accident in previous years and is not present most of the time. She is a Native. She is sold by her father to become a concubine, but with the help of Herman Mellema, has educated herself.