I'm just wondering if Tia may be the trigger for Prof's weakness somehow. We know that they used to date in the past at the very least. It could be that his weakness has something to do with failing to protect someone he cares about (stemming from the deaths of his students), and from what I've seen Tia would be the only living character to qualify as I don't feel that Prof has the same connection with the other members of the Reckoners. In order to prevent his weakness from being taken advantage of, Prof would kidnap Tia and squirrel her away somewhere safe. Then again, I may be completely nuts, but that's my theory.