Joan:
617, 122
Pyungmoo:
395, 1144
Yongan:
935, 1040
Valley of Seungryong:
282, 1455
In order to enter the grotto, you must present Koe-Pung with a Blood Stone, which you get from Seon-Hae's In the Grotto quest.
This NPC is not needed for any quests.