What is the theme in Number 9 Dream by David Mitchell?

Dealing with Loss is the central theme of the novel, as far as Eiji is concerned, is dealing with loss. Eiji has lost his closest friend and family member, his twin sister, and is left with a massive hole in his life. Once he rejects his mother, who is distracted by serious problems of her own, Eiji can see no one and nothing with which to fill that hole, so he refuses to come to terms with his sister's death.