A good question would be: Why does Hannah go by the name Chaya in the past? The answer is that it’s her Hebrew name and the name of the woman she was named after.
Hannah is a young Jewish girl about thirteen years old. It is Passover when the book begins, and she feels bored about her Jewish heritage. Her relatives have all gathered, and they seem to be angry and reminiscent all at once, and Hannah does not know why. All she knows is that she is tired of remembering.
When Hannah gets transported back in time, she realizes that she is being called “Chaya.” The name sounds familiar to her.
Chaya. That’s my Hebrew name, Hannah thought. The one I was given to honor Aunt Eva’s dead friend. Weird. (ch 4, p. 24)
The name is important, because Hannah is symbolically transmitted back in time into her aunt’s friend so she can understand why this friend was important to her aunt. Hannah’s actions as Chaya are brave and selfless. She even sacrifices herself at the end, before she is magically returned to her own time.
Hannah's becoming Chaya means she is accepting her role in her family, and her part in her family's history.