Who is Dzia-Dzia, Michael, Marcy, Mrs. Kubiac from The Collected Stories of Colette and what is their importance?

Dzia-Dzia is the young boy's grandfather in this story. He is a difficult character. When his children were younger, he often left the house for days at a time to return without explanations. Now, older and sick, he entertains his grandson and challenges his intelligence by talking about Chopin and quizzing his grandson about classical music.