I think Bret Stiles is Red John. He can manipulate people; in a Season 4 episode, he tried to convince Grace to join his cult; it seemed fairly easy for him to do though she declined but he was able to "hypnotize" her for a few seconds. He is very smart, he has the means to travel as he pleases, he has connections (as we can see all of the people that "work" for him) in and out of the police and the FBI. There's something really "weird" about him; he's almost likable though he can also be scary. He's not in a lot of episodes but his character reappears here and there with new elements every time. The fact that he's kept in the background shows that there's more to find out about him in future episodes or maybe the producers of the show are trying to confuse the viewers.