Rohde always tells me that he's busy and to come back later. When I do, he says the same. How can I make him join me?
Rohde is a historian (as you're told by various people in Ribble), and it takes a very old item to get him interested. Get the Achilles Sword and take it to Rohde (see the important items list or walkthrough for how to get the Achilles Sword), he'll be a lot more talkative then, and will gladly accompany you to the Taros Shrine to find the legendary Caravan.