I really dont know who is RJ,but i can predict the ending. :P
*WATCH THE PROMO OF S06E06*
We see that house where are suspects and jane will explode.So i think that all suspect will be dead and jane will somehow stay alive(idk how :P)and lisbon and jane will think that RJ is dead.But I think that in episode 06x08(episode Red John) RJ will kill someone important(mby Rigsby idk) to Jane and he will put a smile face on wall where that person is killed.Jane will come to that place and he will see RJ painting a smiley face on wall and RJ will be saying a Tyger tyger poem and then we will see who RJ is and we will be very suprised who RJ is (list is fake i am sure ;P).I think that this is the best ending possbile,but Heller knows better than me.

Sry about English.