In The Devil's Arithmetic, Eva is Hannah's great aunt. Specifically, she is the sister of her paternal grandfather, Grandpa Will. Aunt Eva is Hannah's favorite relative at the Seder, because she shows more attention to Hannah than she does to Hannah's younger brother Aaron. Furthermore, Eva is particularly close with her nephew, Hannah's father. Since Grandma Belle was away at work so much during Hannah's father's childhood, Aunt Eva helped raise him.
Towards the end of the book, Hannah discovers Aunt Eva's true former identity. Before she left Europe, Eva was known as Rivka. She is the same Rivka that Hannah/Chaya switched places with at the concentration camp to save her life. Eva tells Hannah her story of survival and about Chaya's sacrifice. In fact, Hannah was given the Hebrew name of Chaya in memory of Eva's friend and savior at the camp.
When she came to the United States after the war, she changed her name from Rivka to Eva. Many Holocaust survivors changed their names in order to help them start a new life and move past the trauma of the Holocaust. She wanted to forget her terrible past. Yet, at the end of the book, she admits to Hannah that it is impossible to forget.