I am new here and wanted to share my thoughts on who it is, and is not. Huge fan of the show and I am also a writer.

IS RED JOHN A WOMAN?

I tend to doubt this for several reasons mainly that Heller in every interview refers to John as a man. If he used the words Red John when talking about the character, instead of "he" etc. I would buy into it being Kristina. However he said at the end of the day Red John is just a man...and that we might have seem HIM already.

Is Red John Patrick Jane?

Lot's of reason's to think so...great theory and the concept would rock. Jane could have multiple personalities..The Tiger Tiger is all about opposites, and good vs evil, two sides of the coin per say. Patrick has a way with people, he can sort of control them...and so does Red John however, the major problem I have with Patrick being Red John? There are so many people involved there is no way they could convincingly sell it as Jane being split and being both. Heller is too smart to screw this up like that. It has to be someone else.

Red John is small and pettitte...so we can rule out anyone large and tall like Bertram.

Could it be Hightower? Worst theory ever. Makes zero sense, sorry.

Rigsby or Cho? Again, no sense.

Stiles? I just don't see it.

Here is my theory...Red John is someone that is smart but obscure and always has been. However, he is someone that has been around. We may not have always seen him.

My theory as of right now is that it's Ellis Mars

Reason? He is an astronomer. Tiger Tiger has astronomical references. "Roll Tide" is an astronomical event. The Red Face? The Planet Mars (also is last name) has a red face with eyes and a mouth. He is one of the few that has been involved in a Red John case and he is still alive. The writing on the wall "He IS Mar"...perhaps the IS is not  an "i" and an "s" but an L and an S making it "he LS Mar"...Mars is the Red planet. He is one of the few involved in a Red John episode that did not claim to know Red John at the end. He LOOKS harmless (soft). He knew Jane would scoff at him and not take him seriously because of his "gift" so he felt safe. Correct me if I am wrong but didn't Ellis Mars business card have a tiger on it?