You Si is a Goblin Mechanic, who is most skilled at fabricating mechanical items, to the point where he is literally named You Si, and he also has a brother named Gear.
He is the biggest supplier of Airships and their components in the entire black market. His main business is manufacturing and maintaining Airship components, while his side business is alchemy.
Actually, the profession of alchemy is quite broad, encompassing mechanics, potions, material reinforcement, enchantments, and so on—all can be classified under alchemy. Someone like Durken, known as the King of Alchemy, is versed in all of these areas.
But You Si specializes only in mechanics. Airships are large mechanical structures, while components are the small ones. Whether it's Airships or components, if they are equipped with Space Ornaments, their grade can easily be enhanced, fetching a good price.