I think Red John could be an unknown brother of Patrick. Patrick has grown up with the dad, but where is the mom? I suppose the Patrick's mom died in a fire when she was with the little RJ, who believed dead instead survived. This is also the reason why Rj knows about the girl who is a happy memory from Patrick's childhood: RJ was there, with dad, mom and his brother Patrick.


TIGER, tiger, burning bright
In the forests of the night,
What immortal hand or eye
Could frame thy fearful symmetry?

Which better symmetry of a brother? Maybe even a twin one?