Susan Thompson is an unseen character. She is the daughter of Irish American Catholics Ethan Thompson and Elenore Thompson. She died prior to the beginning of the series 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 tuberculosis 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")