Who is Dr. Herbert Kruskal from An Interrupted Life: The Diaries of Etty Hillesum, 1941-1943 and what is their importance?

Herbert Kruskal was a medical doctor and an acquaintance of Etty Hillesum. The first of Etty's letters from the camp at Westerbork was addressed to Dr. Kruskal's sisters. Kruskal had asked Etty to write to his sisters, giving them her impressions of life at the transit camp.