Madame Lavientia is a required npc to unlock the knowledge for Merv, the Calpheon Tailor. She is located in southeast Calpheon, in the Priest District. By speaking to Lavientia, you are in turn, ableto speak with Merv, which will allow you to do the amity minigame to open up her amity shop for Calpheon Noble sets that are craftable.

Just get the quest here from the Choir Leader to find Madame Lavientia.

Location of NPC: Lavientia Batian
from which you can get the knowledge: Madame Lavientia

Quest starts at "Choir Leader" https://imgur.com/gallery/EtS2YsW