Well, the only people that would know his true name would be the people in the room that heard him say it. Plus anyone summoning him.
So, Jehenna, Lenamare, Trisfelt, Elrose, Jenn and the other students. Of which one is known dead (cindered). A few students have not specifically been mentioned as having gotten to Freehold so they are still MIA. That would be the half you can’t confirm.
After that, it would only be people who have had to be able to summon him: Rupert and me.
At least I think. However, if I am wrong, I am sure someone will remind me. o:)
If a demon got a human’s true name, that depends on the demon and the person. Most demons would not be able to do much with it, since most demons are not spell users/direct mana manipulators. E.g. most demons just have intrinsic magic. So the best they could do is sell it to a higher demon, if there was value in it.
As a rule the demons that could do something with it are:
Greater Demons (the ones that are spell users)
Incubae and Succubae (and a few others we will get to at a later date)
Now here is the thing. If an ArchDemon or Demon Prince even knows you [b]exist[/b], let alone knows your true name, you should probably start crapping bricks at that point. They really don’t need your true name to make you very very miserable.