Who is Professor Yun in the novel, Almond?

Professor Yun is Gon's father and Mrs. Yun's husband. Whenever Yunjae visits Mom in the hospital, he sees Professor Yun watching him. Then one day he appears in the bookstore, wanting a favor for Yunjae. Because he thinks Yunjae resembles Gon, Professor Yun wants Yunjae to pose as his son during a hospital visit to Mrs. Yun. He does not think Gon is in any state to visit his mother before her death.