Jonathan Turner was Cory, Shawn, and Topanga's teacher at John Adams High. His authority was sometimes undermined 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.
His appearance also suggested that he was a young man, which was strengthened by the fact that Cory mistook him as Harley Keiner 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.
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).
During this time, he developed a strong relationship with Shawn.
Put off becoming Shawn's legal guardian, and Shawn's dad returned before he was forced into becoming it.
Remained close to Shawn (and to a lesser extent, Cory) even after Shawn moved out.
Was in a motorcycle accident in Cult Fiction. He didn't die, but he was never seen again on Boy Meets World. His final words of the series were spoken to Shawn, "The Center 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."
Was mentioned near Graduation in season 5 by Minkus saying "Hey Mr. Turner, wait up." and running after him in the hallway where if you go down you are never seen again.
He reappeared after more than 18 years on the Girl Meets World episode Girl Meets the New Teacher.
He married the nurse who took care of him after his motorcycle accident.
He is the superintendent of the New York Board of Education, and was the one who hired Cory Matthews as the history teacher at John Quincy Adams Middle School.
He still rides a motorcycle
He quickly realized Farkle is Minkus' son right after meeting him.
He reveals to Maya Hart that Shawn was, and still is, like a son to him.