I see; there's a common theme between the humans and demons that's developing already, and I like that.
The thing is, I think the RP could run into some difficulties if players are expected to play, essentially, consciences of other players. I'd personally think it discouraging to control a practically static character, as in they would never go places willingly.
Your concern about favoritism among players is true, but I will not pretend that this role-play's sign up will be structured to work against that. A truth about role-play is that players choose to work with those whom they are most familiar with, as they consider that person to be more reliable than an utter stranger for a number of plausible reasons. It's also an assurance to the GM that players, be they coupled, are working with people whom they enjoy working with and consider reliable or more apt to being understanding, cooperative, and predictably available. It assures that the role-play lasts longer. That, though, does unfortunately shunt out newer players, and I'm empathetic to that, though there is nothing I can do about it short of forcing pairs, which I know will not work in the long run for this type of role-play as it stands now.
But I should explain how I initially considered the sign-up process:
Students sign up first. With the available students on the roster, those who wish to play as a demon can claim a student in the OOC with their demon application. If everything is up to par in their application, the pair is formed and that's that.
As for
quitting the role-play, since I give the option for players who want out of the role-play to just make a final decision post, either the student finding and performing the successful cleansing rite or a post with a heavy implication that the student carried out an act to end their life, the story can flow sensically and the two players can be on their way. The story, though, does require some dedication, since I'm thinking these decisions cannot be made until seven IC days after the binding. Cast cannot be replaced, either, so when this RP is done, it is done through individual characters' resolutions.
I sort of do want some connectivity between players, otherwise I wouldn't have considered the demon position to be a player character as well, but if people express the preference to just control their own characters and write their story, they may, but that feels too free, and when it becomes too free, the unmotivated become aimless because they cannot keep churning out their character's progression, which is a common situation with sandbox-style role-plays. I want to avoid that.
I like the idea of players signing up with both a student and a demon to play, and their demon possessing another player. My only concern is if one person slacks, that's
two other people they're pulling back, their own student who needs to respond to their demon's player, and their own demon who needs to respond to another student player. This is where that whole player favoritism issue you brought up comes in handy, but then I may as well be upfront about that and say that the role-play is a
closed, pre-invite RP instead of lying through pretense that it's open to everyone when the concern is keeping things afloat with people who have a more active presence on the forums and established "reputation".
Talk about the social politics of a role-play forum. Goodness.
As for your interpretation of the demon mechanic, I want to know how mobile you think the demons are, and how dedicated to the goal you think the demons are required to present throughout the role-play, as well as how consistently. Those are big factors, I feel, between feeling less restricted in that role than the basic description of the mechanic lets on.