Kristina Frye is Red John. She is sadistic and lesbian and likes to spank her victims so long that their back becomes red. Than, she does faceslapping to them, smack, smack smack. After being slapped by her, the victims have really red cheaks.
That's the reason, why she calls herself Red John, actually meaning Red Joanna.
She's intelligent, smart and equal to Patrick and she slapped his family to death.
Would make as much sense, as the actual thing. We will see.