I think that Brett Partridge is RJ:

RJ fooled Patrick trying to make him think that Lorelei was dead, Patrick realized that Lorelei was alive but he didn't show so because he suspected that Lisbon is related to RJ. RJ decides to start killing people close to Patrick, and he kills Leelee, Patrick needs to be sure 100% that Lisbon is RJ's accomplice, so he tells her that Leelee is a happy memory of his, Red John always wants to show to Patrick that he is much smarter than him, so if RJ tells Patrick about the happy memory, Lisbon is with him, because the only possible way RJ could know that Leelee is a happy memory of Patrick is cus' Lisbon told him

Sorry for my English, I've been studying English for 1 year only :D