Given the gore imagery, hospital areas, areas that look like the inside of a human body, and school gives many aspects for theories.
in one event Sabitsuki kills herself, it's hard to see what she's doing but i guess she slits her wrists ( looks like she's eating her hand, probably did it with her teeth ). She wakes up and the computer got bloody. My guess is that she coughed up blood.
So she may have a cardiac disease.
The " Rust " things in the game may present a virus that's killing her, the little fellas that run into you can be interpreted as blood cells trying to protect the body. Some gore and viscera is probably just for the atmosphere, or the producer wanted to be edgy.
Another point to start at is the school. Sabitsuki probably went there, something like a suicide happened and now she skips every lesson.
Maybe she now hides because she committed a crime, like OP said.
The tattoo effect perhaps means that she was part of a gang in school, yes.
The whistle maybe to alarm her group members.
Hood and pipe are obvious.
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