I think that RJ is definitely Walter Mashburn. He's been in two episodes and you get the feeling that you don't see the real Mashburn. You only see his projected exterior and it seems like his interior has some dark qualities. (This is the main reason I don't find Minelli to be a likely suspect - we know his character too well). Mashburn's interactions with Jane are fascinating. They have conversations fully laced with RJ ideas:
Mashburn: If you plan it right, you can get away with murder, maybe next time I'll be the killer, you'll have to catch me, describes himself as vain, egotistical, hates being ignored, cunning, ruthless, and hates to lose - sounds a bit like Jane and we know RJ and Jane are alike, and he brings up Jane's past.
Jane: describes the color red as representing passion, lust, & anger ( be sure to watch Mashburn's slight smile at that - almost like an inside joke because red's his color)
Other things to notice: when you buy the full seasons packs of the mentalist, it's season 2 that's theme color is bright red (1-blue, 3-green, 5-dark red/brown). This is because Season 2 is when RJ is introduced and it's episode 13 (unlucky #) Redline where Mashburn/RJ is introduced. Mashburn constantly wears the color red and also uses RJ's other color yellow. Remember the yellow flowers in the episode where Bosco & team were killed that RJ left for Jane. Mashburn's car that Jane crashed was yellow and the one Jane "chose" for him was red.
In the second episode with Mashburn, we learn that Mashburn had his heart broken by a woman when she chose to marry Yuri instead. This could be what threw him over the edge and what influenced him in how he chose to take revenge on Jane.
Mashburn is the only suspect with the money and the personality to be RJ. He is really the only one that's managed to have a similar effect on Lisbon as Jane does and we know that Jane and Red John are very much alike.
