always thought that Brett was RJ and now i'm even more convinced.
My arguments:
1. He's the weirdo who appears in most RJ episodes
2. He seems to dislike Jane who dislikes him back, and admire RJ's work somehow
3. The voice in the early reasons AND the one on the first episode "Lisbon can't come to the phone right now" is very very very similar to his
4. Lisbon was mysteriously sent to his place. She found him "nearly dying" or did she think. He recited the Tiger Tiger lines, He was behind her when he has been attacked. He could have pretended to be dying and stand up and attack her.
5. RJ usually kills the people he captures, if he left her alive, it's for a particular reason. Maybe because he wanted her to say something that Jane and the other people investigating couldn't have guessed themselves: he needed her to say that she saw Brett Dying. Why is that ? Because this way, we won't need to check his face, if it's really him taken to the morgue. Because, they didn't show us the face of the dead man, did they ? On the 6x06, when Lisbon's at the hospital, you can see a nurse coming in and calling her Mrs Lee. I think it was a clue pointing out the fact that mistakes can be made ! Remember when Lorelei was taken to jail, when Jane and Teresa came for her, there was another person at her place. That's when RJ's friends could be useful, to mislead us into thinking that Brett is dead.
6. On the episode of his last appearance of season 5, it was a RJ crime scene and Brett was there AGAIN but with another forensics man (which was new. I remember thinking, "who's that ? Why is he shown?"). I remember how Lisbon (or Jane, I don't remember) deduced throughout the episode that RJ wasn't alone on the crime, and might have gotten some help from someone else. Isnt't convenient to erase every proof or finger prints when you work for forensics and you're the first person on the crime scene !
7. And finally ... The description of Sophie Miller at the end of 6x02, When she said the words "fearful" or "scared" it clicked in my head. Remember in 6x01, Brett feared Jane. He tended to defy him during seasons, BUT he still fears him ! You know, their relationship is weird. He admires Jane but still feels threatened by him. Killing him in the first episode of the season is a great way of forgetting him but it's too easy ...