Brett Styles, Ray Haffner, Bret Partridge and Lorelei Martins will return in Season 7. I believe that is why the show was renewed (also because Simon Baker has a contract until season 7) and everything will be explained, I see no other reason why we only have 13 episodes. Everything comes to an end.

Brett Styles: First Red John (Until Elliston Farm) and mentor of the actual Red John.
Ray Haffner: Roy Tagliaferro
Brett Partdrige: Actual Red John
Lorelei Martins: She told the list of the suspects to Jane. (As the Stoopkid has said).