He doesn't need to be 4th to 5th level to start with. He could start at a lower level, surely, or perhaps restrained in some form or manner. Neither does he have to be with the party at the Very Start.
Ever heard of the Witness Protection Programme? Perhaps that is why he is stuck out in this minor town on the edge of the universe (whatever). The very fact that organized crime is minimal in this area makes it an excellent place to put someone in the WPP.
Why would they want to take a witness to organized crime along with them? Because they have no choice, obviously. The local Cleric gets zonked on the head, the stooge is the only one who hears evil whisperings, they daren't let him go, there isn't anyone else they trust to take care of him (you could have previously have the town guards glower at the stooge, threatening him, wanting to kill him on the spot), thus it is perfectly clear that leaving their charge behind isn't a safe option.
Combine it with something that lets the Skeleton-to-be provide them insights on the source of the bad nightmare stuff (He hears the whispers even while awake after all). Obviously the Nasty dude also suffers during this whole episode, having headaches, not sleeping, conflicting emotions the whole works.
Just some ideas for you.
I don't think i'd have the guy around from day one, at least not as a prisoner. Perhaps he is just a minor courier or shady burgler, a distant friend of theirs. Have him then disappear for a couple of weeks, then turn up in the Witness Protection Programme, after being involved in some fantastical coup attempt or some such.