Desir’s quests can be found in 3 locations. You have to complete them in order.
2) Port Rosage
3) Gardenia (After retrieving the 1st bead)
The quests are very straightforward and simply require you buying items from nearby shops. This guide is pretty much just so you know where to find him. I’m pretty sure he shows up in Rosage after you doing something in the story but I can’t remember what.