Sounds like an aspect to me, maybe with a Linguist stunt or a Mental Library custom power attached.
There's a sidebar somewhere in YS that talks about personified powers. I guess it might be useful here.
I will have to search for that sidebar. So far, an Aspect does seem the best way to model this, at least until the entity would have/could have built up its own experiences sufficiently to have knowledge or lore to contribute.
The ability to 'borrow' a stunt like linguist or Medical knowledge is the sort of knowledge sharing I envision the entity would be capable of, once the entity has the chance to gain that knowledge itself. That is sort of the crux of my dilemma though.
Use of an Aspect and Fate points would provide for the entity being able to perfectly recreate an event as observed from the boy's point of view, mentally projected into the boy's head. Similarly, if the boy has read a book, the entity knows the contents of the book and can repeat that information back to the boy if/when asked to do so. The eidetic memory belongs to the entity, not the boy (a subtle distinction, I know...) so therefore things which are repeated are done without any emotional attachment to the event.
Where I see the boy potentially 'borrowing' abilities from the entity would be once the entity has learned a particular language, the entity could translate for the boy.
Much like Lash did for Harry when speaking ancient Sumerian to the ghouls and Etruscan in the Deeps, both in White Night.
The key part of the entity being able to translate something would be that the entity would need to become fluent in something before it can translate, it's not a Universal Translator.
The other part of the modeling issue which hasn't really been touched on yet is how the entity should be modeled so that if/when the entity gets passed onto someone else, the things the entity recorded and learned while the boy was it's host can be relayed to its new host. The biggest difference I would expect after going to a new host, would be that if the new host sees something it doesn't recognize (like a gruff) but the entity would have seen and learned what a gruff while in the boy, the new host could ask the entity what it was and the entity could identify it.
That later sort of interaction would suggest to me that the entity should have its own ranks for the knowledge based skills and/or trappings, which result in the entity having a higher skill rank in some things that its host.
Also, any suggestions on what the entity should be?
An angel (fallen or otherwise) is out, since they should already have a significant body of information and the entity very specifically does not. Yet. The Archive portion of Ivey is sort of what I have in mind, but the boy would be like the very first host of the Archive. Plus the entity doesn't automatically know whatever gets written down like the Archive/Ivey does. Not to mention we don't really know much about the Archive itself. A new or 'baby' Spirit of Intellect that is for some reason seeing refuge and a sanctum in the mortal world is possible, but I don't want the entity to just be another copy of Bob. Another possibility which occurred to me is that the entity could be a soul, forced out of its body and needing refuge.
-Cheers