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 Reikugo), using Shidonoko Reikugo 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 Reikugo 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 innocent and harmless creatures or any of vice-symbols or secret bosses in real world Reikugo will ruin relationships with her patients, especially with Huira, in that case Reikugo 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 Reikugo can be caught in some psychological trap, not only as Shidonoko but even as Reikugo herself, and finally judging by Shura ending Reikugo 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 Reikugo 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 Ending Reikugo had a two sisters, it seems that she killed them or Reikugo blame herself for their deaths, her feeling of guilt is portrayed by mangled, disfigured 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.

Artificial Family
In all endings, there's a very clear correlation between the Symbols and people who seem to not only live in the facility but also have some sort of family like relationship between them and with the masked scientist (No. 9) from the endings, and this masked scientist seems to have a similar correlation with Shidonoko, always starting the sequences related to her appearance in endings with her standing by the side of a capsule where it's implied that Shidonoko lives, and with the Parallel ending finishing with her entering said capsule, as well as some minor events showing the scientist in the Apartment's rooms if Shidonoko is caught by certain special chasers.

Chances are, this masked scientist is actually an a artificial being, with Shidonoko being the virtual representation of her consciousness, with her and the rest of the symbols being created by unknown people (the two npcs who appear by the sides of the capsule in the Heart Wound ending maybe?), with Shidonoko's real life equivalent being named No. 9, it's likely that she's the youngest one, and might have been created as a blank slate (hence her name meaning Child of Four Paths and her design being much more plain than the symbols) to "heal" the symbols somehow.

Since Queen is often the first Symbol players defeat, and her defeat gives the player an item called Beast of Shura that makes Shidonoko aggressive when active, the thing Shidonoko had to heal the Symbols of might have been the Beast of Shura itself, with the Shura ending showing what would happen to those who let themselves become fully corrupted by it. Unfortunately for Shidonoko, since her mission to heal her siblings requires her to fight, becoming corrupted to some extent by the Beast of Shura and failing her mission is extremely easy for her.