I don't know who is Red John but i have some clues i don't know how many of you have watched the movie Red Dragon with Hanibal Lector...The movie is based on serial killer character that is trancending and  becoming the seven headed beast from William Blake's painting The Great Red Dragon.In the Mentalist there is and episode called The Great Red Dragon and Red John always recited Tiger Tiger which is a William Blake's poem.Bonus round:The lover of the serial killer Red Dragon is blind and so is Red John's lover.