Hi everybody! First of all I must say that there are a lot of interesting theories here and almost every possible theory is already introduced. I have to ask all the fans to avoid posting already existing theories, because it's very hard to navigate through the whole amount of almost 4500 posts.
But the theory I'm going to introduce is newly generated likely, so here it is. Red John is... Raymond Harker.
Imagine a story of Harkers, who had two children (even maybe twins), a boy and a girl, Raymond and Rosalind (these names are similar enough to be given by the same parents). Once the girl got sick and Harkers asked some healers for help. Unfortunately, the healers (the Janes, father and son) were fake and this led the girl to blindness. The girl was very smart, but mentally disordered and seeking revenge to the whole world. Being unable to act herself, she began to use her brother Raymond.
But once Raymond made a mistake not killing Miranda Roman (Martins), who managed to write something on the floor before death. Miranda wanted to write Raymond, but was powerless, and her message was interpreted as Roy. And genius Rosalind became to an idea of creating some new person, Roy. So she gave a name - Roy Tagliaferro. So that name became some kind of cover for her brother. Later Jared Renfrew, who probably knew about the massage left by Miranda, started writing "He is" (there are two of them, that's why writing "He is" before the name, because there is a female as well), then realized that he's dying and wrote only the next part (He is MON), hoping that somebody could merge the two messages (RAY+MON).
At some point they realized that it would be safer to hide the fact that they are brother and sister (likely twins), so one of them should change the surname. So Raymond Harker became Raymond Haffner (look at the letters, not very hard to fake rk as ffn). By the way, look at their faces, aren't they similar enough to be twins?
So, physically RJ is Raymond, but the mastermind is Rosalind.
P.S.: Maybe at some point Dean Harken was considered to be the brother. Also Grace Van Pelt seems looking like these two, so maybe she is the third child.
