In the book Devil's Arithmetic, what character said this quote?"Mama...why does he bother with it? It's all in the past. There aren't any...

It is the main character, Hannah, who says,

Mama...why does he bother with it?  It's all in the past.  There aren't any concentration camps now."

Hannah is talking about her Uncle Will.  It is Passover, and the family is celebrating the first day's Seder with Hannah's paternal grandparents.  Hannah is a embarrassed and annoyed by her Grandpa Will, who is prone to throwing "strange fits, showing off the tattoo on his left arm and screaming in both English and Yiddish".  This year, when they arrive at the house, Grandpa Will is watching a television broadcast showing "old photos of Nazi concentration camp victims, corpses stacked like cordwood, and dead-eyed survivors".  Grandpa Will is quite agitated by what he is watching, "waving his fist and screaming at the screen".

Although Hannah has heard about the concentration camps her grandfather survived many times, she does not really understand what they were like and why their memory seems to, even now, influence Grandpa Will as it does.  Hannah is twelve, and to her the happenings her grandparents tell about are just stories, history, part of a past she cannot relate to.  It is only after she goes back in time and experiences what they experienced herself that Hannah is able to understand the full monstrosity of what occurred, to appreciate why Grandpa will acts as he does, and why it is so important for her to "remember" (Chapter 3).