Brett Partridge is one of my many suspects: I posted this again has the previous one had mistakes. 

Reason 1: He appears only in Red John cases.

Reason 2: He is strange...

Reason 3: He appears to have a dislike for Jane, however Red John is intrigued by Jane - This could be a act he puts on, so he doesn't give himself away.

Reason 4: We now know that Red John's killings started in 1988 when he was out on a farm, that Visualize owned.

Reason 5: The actor himself was born in 1968, and in 1988 he would have been 20, which is what they said Red John's age would have been. If Brett has been portrayed with the same age I worked how old he would have been 1968 + 40 = 2008 so he would have been 40 when the show started, now I assume that show is set in the same timeline as us, so now it's 2013, that would make him 45, depending on his birth-date. Which fits the profile he would be around the same age as Jane.

Reason 6: He has the same name as Bret Stiles or though he spells it with a extra t. Now we know that Bret Stiles knows more about Red John then Jane does.
What if Red John is his son, as anyone thought of this? I mean I have lots of theories, but this is good.