Who is Idek and what does Juliek explain to Elie about him?

Idek is the overseer where Juliek worked.

Juliek said of Idek: "Only Idek, the Kapo, occasionally has fits of madness, and then you'd better stay out of his way."

Explanation:

Elie Wiesel's memoir "Night" is a recollection of his experience during the Jewish persecution by the German Nazi regime. The autobiography is one of the first-hand accounts of the historical events of the Holocaust.

Idek is the overseer where Juliek worked. Idek, the Kapo is the overseer at the "warehouse of electrical materials" where the work "the work is neither difficult nor dangerous."

Juliek said of Idek: "Only Idek, the Kapo, occasionally has fits of madness, and then you'd better stay out of his way." He told Elie that though Idek may be working for the SS, they are lucky to be working for a much lenient Kommando. And Elie also ended up working in that unit.