I don't know who RJ is but I want to analyse the suspects.
Brett Partridge
He's as dead as Cho's personality. I believe he was a copycat killer and that it was him in the season 2 finale acting as RJ.
Bret Stiles
Stiles is more slippery than Rigsby's hair gel. I believe he indeed knows who RJ is but fears him and that's why he wouldn't reveal his identity to Patrick. He could also be RJ's father and that's why he would be protective of his son.
Reede Smith
He's rougher than Jane's stubble. He doesn't match the physical description we have been given but he is powerful and ruthless. I think people are glossing over him too easily.
Gale Bertram
Clearly he didn't get past the 'act like a tree class' in drama school. He has the power and influence to be RJ but he seems too gullible and obvious as well as being too tall and old.
Sheriff McAllister
He's had more hats than I've had hot meals. From 6x03 I think he is a viable suspect. He's been there since season 1 and it seems like he has a fear of pigeons and maybe a split personality. He also offered jane his friendship and is a good whistler. However, he is slightly too old which could rule him out.
Bob Kirkland
He's more dangerous than a swiss army knife. I personally think Kirkland is after RJ because RJ betrayed him or did something to him many years ago and now Kirkland is out for revenge. It could also be argued that he is RJ's accomplice/red right hand but I think Kirkland's streaky personality suggests he isn't the type of guy RJ would have as his right hand man.
Ray Haffner
He's lifted more weights than Arnie. Haffner is my number 1 choice. He matches the physical description we've been given and he would have been 21 in 1988. Also, he is the only one of the suspects to call Lisbon 'Teresa' as well as being a good whistler. The only reason I would rule him out is because it seems like he's being forced down our throats.
Thanks for reading my theory.