Your soldiers will be the events with more than one page. First page will either have the soldier with something unimportant to say if you just want them there, or will be blank if you don't want them to even appear before you talk to the guy inside. Second page will be conditioned on a switch (I'll call it AskForDemon) and will have the soldier graphics and the events will be where they ask for the demon.
Now, on the guy inside, you'll need to have some condition check if the quest has been done (maybe another switch on that event, but not necessarily), and if it has, set the switch AskForDemon on.
Pretty easy.