Susan Thompson is a character seen only in Nucky Thompson's childhood flashbacks during Season 5. She is the daughter of Irish American Catholics Ethan Thompson and Elenore Thompson. She died in 1884, prior to the beginning of the First Season, which is set in January 1920.
According to her brother Nucky Thompson, she was a sickly child that concentrated the attention of their mother, to the point that Nucky and her other brother Eli would feel jealous of her. She died of consumption when they were children. ("Georgia Peaches")
Following the death of their father Ethan, Eli comments that he is with Susan and Elenore in heaven. ("Two Boats and a Lifeguard")