okay, i know i'm not the only one wondering what the purpose of Yuki-Onna is other than getting the snow effect, and i haven't seen any speculations on her so far, and so i'd like to offer my thoughts. My prediction is that Poniko, Yuki-Onna, and Madotsuki were all friends, and during the winter they were playing around on the ice, Madotsuki went to go see something for a bit and left them alone, and Poniko fell in the ice and died of hypothermia, and Yuki-Onna couldn't help her somehow, and then started pacing waiting for Madotsuki or someone else to help Poniko, and thats why Poniko's house is accesible through a hole in the ice world, and i think that Uboa is a destorted version of Poniko's frozen face, and the sound with uboa is Madotsuki's scream when she saw Poniko's frozen face. i also think that uboas messed-up face is the face Poniko probably made when the ice broke (distorted, of course) and the world you go to when you go close to uboa is probably a drawing Poniko made, and Madotsuki remembered it when Poniko died, but this is just a theory