It's Sheriff Thomas McAllister!  Patrick and him did shake hands in the form of rock, paper, scissors.
If you are going to watch some very important episodes like Rebecca's poisoning and the plot to get rid of Hightower both have one common denominator - a county police. The guy who poisoned Rebecca was indeed McAllister. Of course, no one would suspect the sheriff because only a few knew who he was. Remember Todd? He was also a county police, and don't forget about the guy who kidnapped Jane in Season 1.
This one is a very strong evidence: In the Red Hair and Silver Tape episode, when Van Pelt was about to be harassed, Rigsby stepped in and pushed McAllister. Out of the blue, a police car with its siren, suddenly came speeding towards them(probably one of RJ's friends). Same police car came running in front of Patrick and Lisbon. 

And the " He is Mar" thing ----- Red John planted that cipher to taunt Patrick. It's simply part of: sHEriff thomAs McAllisteR.
Did you guys notice on the season 5 finale --- only the McAllister was given a distinction. Lorelai clearly read his name as " Sheriff Thomas McAllister". The rest were mentioned by their names. Lorelai could have mentioned Director Gale Bertram, or Special Agent Ray Haffner, see the difference?
Maybe the "i" in the He is Mar is not an "i" but a "t". Mind games!

The rock,paper, scissors is one way for RJ to get intimate with Patrick. Lastly, being a county sheriff is a great cover. I'll leave you guys with this Sherlock Holmes quote: 
 "Do you know, Watson," said he, "that it is one of the curses of a mind with a turn like mine that I must look at everything with reference to my own special subject. You look at these scattered houses, and you are impressed by their beauty. I look at them, and the only thought which comes to me is a feeling of their isolation and of the impunity with which crime may be committed there."