What does the nightingale want?

Hint 1 of 15:

I'd suggest waiting until you figure out if the nightingale belongs to 
anyone.

Hint 2 of 15:

Have you talked to the jester in the pawn shop?

Hint 3 of 15:

He'll show up from time to time.  Talk to him to see what he has to say.

Hint 4 of 15:

Eventually, he will mention Cassima.  Find some way to introduce yourself.

Hint 5 of 15:

You've used it before as a means of identification.