*SPOILERS* Story and true ending explained!

[Earthplayer]

I think I cracked the case. It really bothered me how bad/underwhelming the true ending felt at first and there were too many clues which led to nowhere if the "life is a labyrinth and you always die in the end" meta theory many have would be true. But I think I figured it out and all hints, pieces and clues now finally make sense!

1) Who are YOU? - I think you are the mother of the girl. This would explain her deep connection and playful behaviour and would explain why your reflection in the hospital looks very similar to her but not the same. This also explains why the girl would jump on a bomb to protect you. (I doubt any kid would do that for a stranger)

2) What is the labyrinth? - It is the fight of a mother against some illness and shows the confrontations and options she has along the way. And it is depicted through the eyes of the mother and the daughter at the same time - this is why the style often varies a bit - but just enough to make it work if it is a mixed representation of the mind of mother and daughter combined.

3) What happens? - The different endings show different paths of the illness and treatments. The girl sees the illness as something evil and sinister she can't control (the black man) and maybe even projects her feeling of it being evil on the doctors trying to help her mother. The shogi game where you fight the girl is an argument where the girl tries to stop you from getting a treatment which might be terminal (because you look healthy) and if she wins she shoots you because stopping you from doing the surgery is basically like killing you. This is why she cries once she enters the hospital room and sees your situation has gotten much worse. She feels as if she pulled the trigger on you.
If you win the argument you go through with the treatment she shoots herself because it feels to her like you want her to be alone. She feels betrayed. The figure in the background doesn't force her to do the shogi duel - the girl feels like the mother is fine (no threat of death / illness / black mask person) and the surgery might kill her because the doctors said so. The girl feels as if the mother forced this argument on her and not the illness!
In other endings the illness is already stronger when the decision has to be made so the threat of death of the mother exists (black mask person forcing a duel/argument instead of the mother).
The syringes endings are for a different treatment that involves some sacrifice from Katia (some organ transplant maybe?), but this treatment is also safer for the Player. That's why Katia is also in a hospital bed, as represented by the wheelchair (she normally doesn't sit in a wheelchair - you always do hinting at you being in the hospital the entire time). The more Katia sacrifices, the less healthy Katia becomes. Depending on how much the mother fights for survival by taking her childs transfusions/donations/whatever, the worse the kid will feel. If she decides to stop (2 syringes inside the girl -> already took a lot) once the girl feels really bad, the mother dies, but the child has terminal damage (maybe a surgery went wrong and she now has brain damage) If she stops after the first surgery/donation the child is fine but very sad that she couldn't help her mother. If the mother doesn't accept her daughters donations she dies immediatly as the treatments for her are not enough and the girl will be fully healthy but feel like she should have done more to help her mother. (taking from her once is the best ending here as the daughter will feel as if she has done everything she could). If you take the daugthers donations the entire time you actually do survive but become "the evil" (the illness is portrayed as the masked person, now you have a masked face, too - you are now also something evil) because you took the life of your daughter to survive yourself.

Also the lamp makes sense at that point because it is supposed to be the IV drip, and Katia is waking her mother up by disconnecting her from the drugs because the mothers eyes point at the drip several times (shown as a laserpointer in the game) so she can try to communicate with her (apparently arms are broken and she can't talk right or at all because of some accident or the illness, that doesn't matter that much for the story though).

All the different "endings" suddenly make sense. Every ending is some kind of decision or new treatment they try to save the mother. The girl is trying to protect her mother but it is the same the other way around. The true ending simply shows slices of the life of the mother. The board game is full now - the last decision has been made and no matter what survival was not to be. The mother sees her life flying past her and dies. Depending on what you did the girl will be fine, be mentally broken, physically broken or even dead. This is why it says "Only you can save her...". The mother must stay alive long enough to give her daughter comfort and prepare her for the death but must not allow herself to make decisions which enlengthens her life by destroys the life of her daughter.

It's the struggle of a terminally ill mother to save her daugther from breaking down while fighting her own inner demons at the same time. It's complicated and hard, that's why there are so many puzzles. The last labyrinth is the last struggle of a mother to figure out a way to protect and safe her daugther.