Who is Dzia-Dzia, Michael, Marcy, Mrs. Kubiac from The Vintage Book of Contemporary American Short Stories and what is their importance?

Dzia-Dzia appears in the story, Chopin in Winter. He is the young boy's grandfather and a somewhat difficult man. When his children were younger, he would leave home for days without explanation. As an older man, he spends his days entertaining his grandson and challenging his intelligence. Most of their conversations include talking about Chopin and quizzing the boy on his knowledge of classical music.