I think it is the DA Osvaldo Ardiles.  He is not quite six feet tall, not muscular nor flabby, he has the same build as the Red John who saved Jane from the fake red john movie makers, when Jane was tied to the chair, in "Red Sky in the Morning."  He is intelligent, and has access to restricted police computer systems and case information. I think it could be him. He would be in a perfect position to see if any warrants were to issued for any of his disciples. He was one of the four suspects in the  murder of Todd Johnson who was set on fire in custody at CBI, and the way that Rigsby tried to give the hint  to Ardiles as to where Hightower was, it was lame and besides O'Laughlin had received a better tip from Rigsby. As a DA, he can talk to any person involved with the Red John cases and not seem suspicious.