I've just posted a theory accusing Gale Bertram. I'm very convinced it's him.

BUT, I can not stop thinking about Kirkland Brothers.
1. Why Bret Stilles protects Red John? My only explanation is that Michel Kirkland was Stilles's pupil.
2. About the scene in which RJ cathes Lisbon (6x01 "The desert rose"), maybe RJ had caressed Lisbon in the face to show his empathy on her alcoholic father.

Bruno Heler is so genius!
I hope you understood, my english is not good enough.
Thanks!