Here's my theory about RJ. I am damn sure that RJ is going to be PJ himself or PJ's brother.
1. Their voice is similar. Watch this:
http://www.youtube.com/watch?v=OUikVzxQuuY
2. Red john smiley resembles with PJ's face.
3. Morale behind 'Tiger Tiger burning bright' poem as said by Cho in Season 3 last episode: Good thing can't exist alone. You can't have life without darkness.
"Tiger, tiger, burning bright In the forests of the night, What immortal hand or eye Could frame thy fearful symmetry?"
Hence, words "tiger tiger" or "symmetry" suggest that RJ can be the dark side of PJ or it can be PJ's brother of almost same age, same intelligence, same psychic nature but having darker side.
Hence, RJ is either PJ having a hidden dark side or someone like his brother who is as clever as PJ but has dark side associated with him.
But as Lorelie said, they have shaken hands. As one cannot shook hand with himself, I think its gonna be PJ's brother.
