Mrs. Stern is the woman in charge of the Stern Boarding School where Eva was a student. She is a despotic person who runs the school with an iron fist, and she seems to dislike Eva in particular for her delinquent behavior.
When Don Wei first brought Eva to the school in 2072, Mrs. Stern came out to meet them. She put on a show of friendliness in front of Don, but as soon as he turned his back she dropped the act and revealed her true, unkind nature.
Ten years later, Mrs. Stern hasn't changed a bit. When she first appears in A Fresh Start she is punishing a student for being late by making him stand with a stack of books on his head and holding buckets of water in both arms. When she hears the sound of an engine starting outside, she knows that it's Eva. She berates her assistant for not dismantling the girl's rocket-seat and orders him to stop her, but he fails and Eva escapes the school.
In Optimised Like Ondai, Mrs. Stern appears when Don Wei calls the school from Oban. Before he can say anything, she yells at him that boarders cannot receive calls at this hour. When he asks to speak to Eva, she scoffs and assumes that he's calling to complain about the girl committing some crime. When Don tells her that he is Eva's father, Mrs. Stern is surprised and quickly tries to act polite, promising to find his 'sweet Eva'. He is confused, and she tells him that Eva was up late on her birthday waiting for him to call. The connection is lost after she says that Eva has run away from school.