Who is Dzia-Dzia 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 difficult character. When his children were young, he would leave the house for days at a time without explanation. Now that he's older, he entertains his grandson and challenges his intelligence by talking about Chopin and quizzing his grandson about classical music.