Who is Halmoni from A Step from Heaven and what is their importance?

Halmoni is Apa's mother and Young Ju's grandmother. She teaches Young Ju about God, Heaven, and prayer. Young Ju loves her Halmoni, but is forced to leave her behind in Korea when her family moves to America. Ten years after their move, Halmoni dies. The family cannot afford to return to Korea to attend her funeral, and Apa knows that Halmoni would not want him to borrow money to return. Halmoni's death is hard on Apa, and not being able to attend Halmoni's funeral makes Apa feel more like a failure.