Onsen-san (Mr./Mrs. Hot Spring, おんせん-さん) is a NPC that can be found in the Hot Spring House in the Wilderness. Its appearance can only be seen by using Lamp effect because of the darkness of the house. Onsen-san is sitting beside a hot spring pool and has a pipe from its head absorbing the spring water.
It only reacts to the Knife Effect. Using the Knife Effect on it will change the color of their head from red to blue and it will no longer imbibe the spring water, as if it has died.