I think it's Robert Kirkland. We saw him in 4 episodes only, and yet each of those four brought an important twist to the story.

He was the person in the car at the end of the episode 5.05,
he was there when Lorelei escaped,
he prevented Jane from driving in the ambulance in 5.16. and thus possibly finding out the RJ's name.

I'll see if I can find other indications, but my hunch is that RJ might be Robert Kirkland.