It's Kristina Frye.  It hit me when Lorelei, in her video at the end of season five, states that Red John is a real psychic, and that Jane should just accept that.  That is what Kristina said, of herself, every episode that she was in during Season 2.  I personally don't believe in ESP, and I doubt the show's writers would have Red John be an actual psychic, so I think that Kristina/RJ uses the same devices to perceive things that Jane does.  I cemented in my theory when I thought back to how Kristina was able to hide herself from the limelight, by faking her own memory-erasing/psychiatric coma type deal.  She successfully became forgotten and out of sight by everyone.  She was so close to Jane in Season Two, with the kiss on the cheek, and the date - it's so eerie.  And she has red hair.  I also believe that Jane's psychiatrist that died was working for Kristina, along with Brett Partridge and many other disciples.  My guess is for Kristina!!