Jonathan Turner was Cory, Shawn and Topanga's teacher at John Adams High, having ran away from a rich life-style in Connecticut to be a teacher. His authority was sometimes undermined of by George Feeny, due to the fact that he had new and fresh ways of dealing with students, as opposed to the conservative ways most other teachers have used. He often says, "Are you kidding me?"
His appearance also suggested that he was a young man, which was strengthened by the fact that Cory has mistaken him as Harley on his first day of high school. Turner's teaching methods were often questionable, but they have always brought desired results and taught the characters many valuable lessons.
In one episode, we learn that Jonathan's parents were rich, but he decided to run away in order to fulfill his dreams of becoming a teacher. Throughout the time he appears in the show, Turner develops a strong relationship with the main characters, particularly Shawn Hunter, who lives with him for a brief amount of time while his dad is away. After this, he remained close to Shawn (and to a lesser extent, Cory) even after Shawn moved out.
He was a credited as a regular during season 2-4, but he appeared only sporadically during season 4, and was credited with a guest appearance in the series finale, but appeared only via flashbacks. His final real appearance was in "Cult Fiction", where he was involved in a motorcycle accident. He did not die, but was never seen again. His final words of the series were spoken to Shawn, "The Centre is filled with lost souls who have no belief system. Who are targets for some guy to bring over to his way of thinking. The Shawn Hunter that I know is one of the most unique individuals I've ever met. And if you let this place take away who you really are, Shawn, then you've made the worst judgment you can make. You made the judgment." He was later mentioned near Graduation in season 5, by Minkus saying "Hey, Mr. Turner, wait up." and running after him in the hallway where if someone goes down, they are never seen again.