I believe it is Haffner. The #1 reason is because Lorelei Martin said "Im surprised you did not become life long friends", she also talked about how similar they are. I would suspect that RJ is not overtly showing his true colors but there is one place that is almost impossible to hide and that is the heart.
Haffner and Jayne are both in love with Lisbon. If RJ killed Partridge and spared Lisbon, the most likely reason is due to love. Also Lisbon was going to go out with Haffner and definitely has feelings for Jane. It is like Lisbon is attracted to a type, a type RJ and Jane are both.
Also, I keep asking why not kill Jane and other than toying, it seems that RJ is at war with Tyger Tyger and want to kill all the members and is using Jane to help find them. Haffner has ties to the Barn and vizualize. It makes the most sense to me.
