Come now. Since the Giant does not actually count appearences, the Mystery Ally is obviously going to be Belkar or Redcloak.
That's why I think the Mystery Ally is going to be either an agent of Tiamat (yeah, I know, but I'm not giving up on this so easily) – which means access to the Oracle and the infrastructure of a church – or the IFCC's vessel (the Directors know where the Order is and it is trivial for them to transport someone there).