Hades is, in fact, the Ultimate Synthesist referenced by the game's manual as well as a few characters you talk to in the game. He can only be found in Memoria (Disc 4) and is very well hidden. In the room where you're in the ocean, go to the right of the stairs. There's a little "hidden" area. Hades is back there, but there's no popup icon... you need to run around pressing X against everything until you find Hades. Challenge him to a fight, and after you defeat him, he will synth items for you.

Hades, as a boss, is slow and not very tough. A simple strategy is to rely on physical attacks with three people, and cure with the fourth. Hades almost never does anything really damaging to all your characters at once (other than Cleave, which only takes off about 1000-2000 HP). Be sure to protect against Freeze as well, with Body Temp.

After many attacks, Hades will fall and items, such as the Pumice, Tin Armor, and the Save the Queen will be yours to synth. (The Hammer item, if you obtained it from the Stellazzio sidequest, will be used in several of these items.)