Who is Mrs. Gruber from The Cage and what is their importance?

Mrs. Gruber is the landlady at the home that Riva and her family had lived for years prior to the German program to kill the Jewish population. The woman, though not a Jew, had been very close to Nacha prior to the German harassment of the Jews but was among those who raided the Minska home and denounced the family, leaving them literally to freeze and starve.