Red John has been very subtly been introduced to us in season 5 episode 5 whee a split second shot of him was visible at the end of the episode. He reapeared as a federal agent in season 5 episode 7 and revealed to be Robert Kirkland, homeland security.
He took special interest in Jane 9 years previously whne he joined the CBI team,( please see flashback episode where the FBI agent makes a call to Virgil to cooperate on the Red John file ) and when he met Jane he called him by name and shook his hand and proclaimed they never met but knew him. Also Lorelei mentioned they shook hands, Jane and RJ.
But the flashback did it for me, the purpose of that whole episode was to reveal Red John
