Monsters and Other Childish Things RPG.
I hadn't heard of this yet, but after a quick google search I found the quote
Can you make it through a school day without having to explain why your monster ate the substitute teacher?
and I definitely need to look into it
As far as an adult character like this is concerned, it is a tough one. It all depends on how much of an impact you want the fey friend to have on the game.
A simple aspect like "my imaginary friend" could be invoked to aid in any task and could be compelled if the fey is trying to eat the substitute.
If you want more than that, and specifically if you want it to be reliable, you would probably need a bond of some sort, reflected by a supernatural power. Maybe something like the righteousness power from a true believer, only instead of using conviction for anything, you can use your imaginary friend to do the task for you, at his skilllevel (or you use your conviction or contacts to do this, and the fey friend is simply the explonation, why you can do this).
If he is a full fledged ally, You should treat him like that, bargain with him for tasks he might do for you and such. Give him aspects that represent his loyalty to you and likewise have one or two of your own to represent that bond.