I don't have a clear vision of who it is yet.  But, it deserves to be pointed out that Jane doesn't shake hands with people very often.  In fact it is rather infrequent.  Also she said, "I'm suprised you didn't become the best of friends from the moment you shook hands."  To me this indicates that they did not become the best of friends.  So whoever Red John I'm assuming it's one of the characters that is someone:

- Jane HAS shaken hands with
- that Jane doesn't get along with
- that has been on he show long enough for a relationship or lack thereof to develop between Jane and Roy.

And as a sidenote ... Roy means 'red'  HA  isn't that brilliant?! I googled it.  This show is one of most well written shows I ever!