He's in a room at the end of a hallway leading to just that room and a dead end in (SPOILER) Hall of Effulgent Light (farthest south area of Necrohol) .

However hes not listed on your map with a mechant key and he doesnt appear to be in the room, you have to (SPOILER) look around the edges of the room and in one corner you will find a "!" sign, he pops out when you click it .

He really does have some cool stuff, especially towards the end of the game when (SPOILER) Sky Fortress Bahamut appears, such as the magic Scathe .

That was a lot of spoilers, but best to be safe :P