Ni No Kuni 2 Walkthrough for the solution to Sin Gul's full name in Side Quest 159 A Man of Many Names.

Location: Capstan-upon-hull (quest icon on map)
Requirement: Completed the Story. After beating the story (after the credits), choose to save your game. Now your save game will get a gold star on the right side. Load that save and you'll unlock new endgame content including this quest. Only after finishing the story it will appear!
Quest Giver: Sin-Gul

Sin Gul is a very important character. His skill "Singularity" is needed to research the Level 8 improvements of Armor and Weapons.

Sin Gul's full name is: Sin-Donwaribihapi-Tstaykenmayt-Haoozbiznis-Sbinawile-Watsamata-Gul