I believe that RJ is Brett Partridge.
(a lot of theories already explained why in details so I'm not going to write the same things)


however, the anagram of some names in the suspects list give interesting results:


Reede Smith ---> THE RED IS ME (heller said we'll be disappointed...)

Reede Smith ---> HE'S RED MEIT ---> HE'S RED MATE (?) (RJ has a good friend in the FBI)

Bret Stiles ---> BRETTS LIES (brett partridge's lies)

in BRETT PARTRIDGE you can find the words RED TIGER PART (tyger, tyger....)



In the season 5 finale there's a scene where Jane asks Brett if he was sure that the killer wasn't RJ. Then you see partridge answering, but if you notice there's the bloody red smile behind him...

it might be a coincidence... but I think they shot the scene that way on purpose
all you see is partridge talking about RJ and the red face behind... it's like heller says "the answer is in front of you jane, can't you see it?"