I am 75% sure that Kristina Frye is RJ for many reasons:

1. She is the only smart person that can be compared with PJ, even though she is pretending to be a psychic, she is intelligent and may be RJ.

2. In season 1 red john was known to be a male when the dead guy drew with blood on the wall " he is man", RJ wont let such thing happen so he or she would write it to confuse the CBI.

3. When Kristina talked about RJ on live TV and then put to protection, she disappeared under unknown conditions and it is possible that she escaped by herself because it is hard to kidnap without attention.