Let's assume Haffner is RJ, for the reasons stated in other theories. I think someone here also said that Heller said that PJ will face a dilemma when he catches RJ- what to do. My theory is that when PJ catches RJ (lets presume its Haffner), Lisbon will be taken by the "Tyger, Tyger" group and RJ will be the only one who could help, so PJ will be faced with a choice between saving his friend or getting his revenge. Also, remember when Bertram said that he wants to be there when RJ is caught? I reckon this is because RJ knows about the "Tyger, Tyger" group (which I think Bertram is a part of), and he wants to kill RJ before he reveals the secret.