Kristina Frye.

She's the only living "witness" of Red John. We just assume she went to the loony bin after Red John "did something" to her to cause her to become "crazy". I'm pretty sure that PJ would have the skills to get her to snap out of it... if she was, in reality, out of it. Wasn't there another episode where PJ helped a victim who went through heavy psychological trauma to finally remember?

We know that RJ killed PJ's wife and daughter because he was offended that PJ would be so bold to pretend to be a psychic in public. Frye is challenging Patrick. Her skill is so good, that he just can't figure out how she does it. She even seduces Patrick to the point where he seriously considers moving on from his dead wife to start a relationship with her. "Red John" is obsessed with Patrick. "He" wants to replace his family with "himself".

*She has a deeper voice for a female. Watching the video where Patrick is tied up in the chair... could possibly be a woman's voice pretending to be a man.