My wife and I both are convinced that it is Kristina Frye and have been for a long time. I am glad to see that others are also thinking along the same lines. I am hopeful that the writers won't see these theories and go off on some BS tangent. As stated in other theories:

She knows far too much about the wife and daughters murders, Jane, who doesn't believe in psychics, seems strangely ignorant of this.

Red John typically kills women and then paints their toenails, I hardly believe that a male would do this.

Kristina is conveniently knowledgable about far too many of Red John's murders.

As previously stated feminine voice.

Writers seem to insist on using the terms "him" or "he" and seem to high light this.

As previously stated has access to high value people to use in her web.

I don't know if she has ability to hypnotize (I can't remember seeing it) but with her background, it would be useful in turning said high value targets.

This are my thoughts, let me know if they are yours.