Who is Jo-Beth from The Great and Secret Show: The First Book of the Art and what is their importance?

Jo-Beth is Joyce and the Jaff's daughter. She is very close to her brother, Tommy-Ray, but he has a dark side that she finds unsettling. After falling in love with Howie, Jo-Beth drives him away, feeling as if their love is partly to blame for the troubles in Palomo Grove. Jo-Beth abandons her brother after discovering he killed their mother.