I just watched the Red Barn episode, and now I think it's Ray Haffner.

1. He fits the physical description Rosalind gave, if we assume it to be correct.
2. He is on Jane's list of suspects, if we assume that to be real, complete and correct.
3. He fits the age group, as the "kid" who painted the smiley in the barn.
4. He's a member of Visualize and clearly RJ has some associations with the group- for example, Brett Stiles knew where Kristina Frye was.
5. He offers Lisbon a job and tries to make her detached from Jane, tries to make her think he'd leave. 
6. The most popular suspect is Brett Partridge, and he just seems too obvious, and too "goofy" to be RJ.