Kataribesou ~Ensouki~ (かたりべ荘～淵藪記～)/theories

Scientist's Treatment
Judging by Abyss Bush ending Shidonoko is something like a virtual or artificial alter ego of one scientist, known as No. 09 (probably sounds as Kugo or Kureigo), using Shidonoko Kureigo treats a psychological problems of her patients, problems or psychological sicknesses of patients is represented by symbols in that world of Shidonoko, and everytime Shidonoko defeating symbols she keeping their forms as her effects, that means Kureigo making something like a cure for her patients, which also helping to Shidonoko herself, main patient is Kureigo's husband, if player will fight with Tsutsukura too long there will be revealed his true name, Huira, small creature with Tsutsukura known as Yayakura can be an artificial embodiment of Kureigo and Huira's child, this child even have his or her own room in Unopened/Tsutsukura's world, but where doctor can heal doctor can make situation worse, if after getting all symbols Shidonoko will kill inocent and harmless creatures or any of vice-symbols or secret bosses in real world Kureigo will ruin relationships with her patients, especially with Huira, in that case Kureigo will realize that if she don't have a dear people in real world she can be happy only in that artificial world as Shidinoko, judging by Heart Wound and Back Heart Wound endings Kureigo can be caught in some psychological trap, not only as Shidonoko but even as Kureigo herself, and finally judging by Shura ending Kureigo in form of Shidonoko can be corrupted, corrupted by a power and her goal to became a strongest being in that world, demigod "Asura", but in fact after getting Asura's title Kureigo refusing from real life and in real world she stay in some stasis state forever, fate of Scientist No. 09 also known as Child of Four Paths Shidonoko is depends from Player's actions, from your actions.

No. 09's sisters
Judging by secret event before Heart Wound Endinlg Kureigo had a two sisters, it seems that she killed them or Kureigo blame herself for their deaths, her feeling of guilt is portrayed by mangled dusfigured or distorted appearance of her sisters, because one person have a long hair and second have a haircut in pigtails, just like No. 09's sisters.