I'm not sure how to answer this without putting spoilers into comments!
I just know when i go to the house you normally go to it tells me i have to find her because she isnt there
If you got the answer, I'd love to know as well, I can't find her either.
Also, does anyone know how to get the ending with fief ? I couldn't figure out how to get it myself...
I still haven't found her
Look at the bench west from Kristine's house, you will find a letter in late part of the game.
