I don’t know who is RJ. But I think we have to focus on RJ appearances.

Let’s take a look at 2x23 (Red Sky In The Morning) and 6x01 (The Desert Rose), when masked RJ appears and talks to Jane.
First of all, look at his body, watch his legs and feet (2x23). We can already exclude Reed Smith, Bret Stiles and Gale Bertram (pay attention to the legs!). 
Focus on his voice and the vocal timbre, try to imagine the remaining suspects saying those words.
This sentence is very important: "Sorry Patrick, Teresa can't come to the phone right now..." (6x01).
We can exclude Thomas McAllister (definitely not his voice), Brett Partridge (he's dead, and btw Jack Plotnick said that his voice was never used for RJ), and Haffner (the voice doens’t match – I know he’s the only one who calls Lisbon by her name but it doesn’t matter).

Now take a look at these 2 episodes: 

Ep 5x18:  “Behind the Red Curtain” 
- Kirkland talks to Jason in the hospital. Just focus on his voice when he’s whispering. 
- Jane arrives but it’s too late, then he asks Kirkland what happened. And Kirkland says “Patrick, I’m sorry…”. Compare this sentence with “I’m sorry Patrick..” (6x01). 
The similiarities with the masked man are fairly evident. Same timbre, same pronunciation.
 
I’m sure that Kirkland is the masked man. And I know that many of you will say Kirkland can’t be Red John, and you are right. 
The masked man, in fact, is NOT Red John, but his right hand. His Red Right Hand.

Ep. 2x08: Minelli speaks to journalists regarding his agents deaths. Right after that, Lisbon goes to talk to him, and he acts like he’s waiting for someone. In that moment you see a well-dressed black-hair man at the gate, he’s just arrived with a limo and Minelli is waiting for him. 
He’s Kirkland. 
The name of this episode is “His Right Red Hand” and this is the first time you see Bob Kirkland in Mentalist, right hand of RJ, arrived to keep an eye on the situation and sacrifice Rebecca in the end if she will get caught. 

Bob Kirkland is the most important RJ disciple (remember when he said at the hospital “I’m a friend. Let's face facts Jason. If you recover, and live, you'll spend life in prison for murder at the play thing of monsters. I just hope, that if I am ever in your shoes, I have a friend close enough to do this for me”). 

He did murders and stuff for his master, he seduced Rosalind Harker (the description fits).He was the patient of Sophie Miller. 

Sophie, in Black-Winged Redbird (6x02), said that RJ had probably a real phobia: Kirkland touch two times and polish off Jane's suit, in Behind The Red Curtain (5x18), without a reason. This is called Rupophobia, and also with other names, and is the phobia of dust and dirt. She said also that RJ is a good whistler: many suspect are seen whistle in this episode; but, for an istant, in Red Letter Day (5x19), we see Kirkland whistling in the office of Homeland Security.

Conclusion: 

>  Kirkland is RJ right hand. 
> Partridge is dead. Don’t know if he was involved.   
> Stiles and Jane are almost friends (they helped each other), and he knows information about RJ (for sure he’s not him).  
> Haffner – he can’t be RJ.   [RJ would never: Admit he’s from visualize - Not answer when Lisbon asked if he was at the barn - Said “I like You” to Lisbon / He’s neither rich (private agency of investigations – money from Visualize) nor powerful.]   
> McAllister and Reed Smith – they are involved in something but I don’t know if it concerns RJ   
> Bertram – my bet is on him. …….I feel some weird vibes…..   :)