Ni no Kuni 2 Quest 159 can be located in Capstan-Upon-Hull. You can't access it until you've completed the main story.
Called ‘A Man of Many Names', it can be picked up from Sin-Gul. He cannot remember his full name. The mission reads:
“A mysterious man has washed up on the shore in Capstan-upon-Hull, and he seems to have forgotten his own name - or most of it, at least. It's up to Evan to ask around and see if he can fill in the blanks, but it soon becomes clear that it's much, much longer than the average moniker”.
So, in Ni no Kuni 2 Quest 159, it's your task to work out Sin-Gul's full name. As a reward you get 9650 XP, 3810G and the Great Sage's Secret.
What is Sin-Gul’s full name in Ni no Kuni 2 Quest 159?
You'll have multiple answers to choose from, and the answers, in this order, are:
Yes, Sin-Gul's name in Ni no Kuni 2 is a big one! And that's it! You've complete Ni no Kuni 2 Quest 159!