Who is Harry Gruber from The Cage and what is their importance?

Mrs. Gruber's grandson, Harry, was the same age as Riva. and the two were very close. As an only child, he spent a great deal of time with the Minska children. When attacked by a group of people and accused of being a German spy, Nacha stood up for Harry, but he later joined the Nazi party and was sent to the German front.