So who's Storyteller?

[Shadenuat]

I wonder if you bring all relics to him you learn some more of him, but during endgame he seems to have some serious powers.

Don't think he's in original module, and I don't think anyone from First World book matches him either.

There's Count Ranalc, who's things are exile and betrayal, but seems different.

Magd and Ng I think we see in visions during Nyrissa trial, and Shyka we deal with for Crown.

So not sure if he's supposed to be all mysterious or match someone from Pathfinder.

[Lethe]

I always thought he was lantern king and thats how he knows the story of all those items because he was present in stolen lands all this time.
though doesnt look like it seeing the ending (but it might just be)

[Zaltys]

I think he's just a new recurring character that Owlcat made for the collection-feature. If they ever adapt other adventure paths, I expect him to show up in those too. After all, the ending says that he left the lands, but his story isn't finished.