It's RON. R...ed J...O...h...N.
As Heller said...Red John appeared sometimes, but never obvious. There from the beginning...always able to get all informations. Someone you'll never think off. Take a look at the latest episodes...Ron is there more often than ever. Last Episode Jane asked him:"Did you see Lisbon?"...he answers, walks away fast, while speaking a little like Red John. Even in the new trailer you can see him...

For me it's a teammember in the series from the very beginning and the one which role is written to be the biggest surprise at the end.