I think that a lot of people assume that Red John is a man and I think it will come back to surprise everyone that RJ is a woman.  First, when Patrick first meet Rosalind, she was serving him tea from a tea set that later shows up on a crime scene where RJ was watching Patrick from another room via video camera. 

And then in the episode where Rosalind calls Patrick after she went with him to identify RJ's body because Patrick thought he had shoot him and she told him that the dead man in the morgue is not RJ, she was the one who also have the opportunity to meet the morgue attendant whose dead body later turns up at her closet after he was questioned by FBI Agent Darcy.  After Darcy questioned the morgue attendant and found out Patrick brought Rosalind to identify the body, Darcy went to see Rosalind and she told her the body in the morgue was not her friend Tagliaferri.  My theory is that when she called Patrick and told him that her friend is back for a visit and it's all just a misunderstanding, we did not see the face of the person sitting on the couch, but I believe the person on the couch was the morgue attendant who does not know that he is sitting with a stone cold killer.  When Patrick showed up at her house after she called, she was playing the piano and claimed that RJ left, but then they found the dead body of the mortician in her closet.  I think she was the one who killed him and she has continued to deceive Patrick because nobody will think that a blind woman is capable of what being RED JOHN.