Brett Partridge was "eliminated" from the show as the first of seven RJ suspects to throw the audience off due to the fact that he was the most compelling and probable RJ. The most compelling evidence is RJ's unusual voice - it's unmistakebly Brett Partridge's voice. There are many other clues that you can read in other posts that would lead you to believe without a doubt that Brett Partridge is RJ (his last name (an allusion to William Blake's poem), his involvement in the pilot episode and other critical episodes, etc.). Brett Partridge and Agent Lisbon have had several moments together (in previous episodes and in his final episode) which proves they were in collusion together. In the bone investigation scene, Lisbon can be clearly seen to be giving eye signals to Partridge that Patrick Jane is on to him.
The best clue is Brett's last words ("Tyger, tyger..."). The poem goes on to state, "What immortal hand or eye Could frame thy fearful symmetry?" Symmetry, as in there are two (twins). There are two "tygers". My guess is that Brett has a twin brother.
