Who is John Dunbar from Faithfull and what is their importance?

Marianne described her first meeting with John as love at first sight. She writes, "when I saw John it was one of those things where everything else just pales around you." The couple went onto to marry and have a child, but Marianne left John after getting involved with Mick Jagger, taking their son and moving into Jaggers home. John tried multiple times to get her to return home without success. He eventually divorced her and got custody of their son, Nicholas.