Sion is interesting in that Cesare can tell they have things in common but also knows he's not telling him any of the juicy parts. He doesn't trust him because he hasn't given him any footholds, so to speak--it's just a sheer wall and Cesare likes having little cracks to dig into. On the other hand he doesn't distrust him because, again, he sees no reason to. He is uncomfortably aware that he has told Sion more than Sion has told him, but he reasons that it is ok because he hasn't said anything particularly important anyway and, besides, someday this will all pay off and he'll get the information that he's so pinged by.