This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
Sabitsuki is a cyborg.
Living in the future, after a horrible accident she underwent an Unwilling Roboticisation. Horribly traumatized she retreats back into her room. Through robotic implants she can interface with her computer, entering a trance-like state in which she can escape reality into a dream world.
Although it looks more like a nightmare so maybe there's a computer virus which slowly effects her brain too like it did to the computer, there's a supernatural entity inside the computer (see WMG underneath this one) or it's her own traumatized mind which corrupts her computer-dream world.
Sabitsuki's computer houses some sort of supernatural entity. (Plenty of unmarked spoilers!)
It's never explained just what happens when Sabitsuki sits down at her computer. Does she doze off? Space out while browsing? Surf really disturbing websites? Or perhaps it's something much more sinister: .flow (though this may not be its real name) is actually an intangible demon or monster that is drawn to humans that have Sabitsuki's illness and interacts with them through their favorite pastimes - a computer in Sabitsuki's case. It is able to draw the user into a trance, and opens up their mind to explore their pasts and fears, the whole experience coming off as a really strange and disturbing dream. Furthermore, the user will always feel compelled to return, their will to leave their homes weakened by their disease and .flow's influence.
The effects are representations of Sabitsuki's inner self, and reminders of her past (the uniform and black hood, and possibly the steel pipe, handgun, and whistle), fears for her own future (the ghost, viscera, corpse, tumbling doll, gas mask, and possibly the dress), things she enjoys (the cat, broom, headphones, and possibly the diving helmet), or simply random moments of make-believe (most other effects). They could be considered a symbol of her remaining mental strength, as .flow ultimately wants her to give them up.
A few, like the cat, ghost and broom, let Sabitsuki travel through her mind more efficiently, inadvertently letting .flow get a better look, as well.
Characters like Smile are memories of former classmates, horribly twisted by .flow. The doctor and nurse were the ones who oversaw her hospital stay, while the maids may represent other caretakers, wearing gas masks (medical masks in Sabitsuki's real memories) to keep themselves from becoming infected. Some, like Oreko, are the few that Sabitsuki is managing to keep mostly intact.
The Kaibutsu are .flow's way of showing what eventually happens to anyone who interacts with it. Should Sabitsuki attack the Kaibutsu in an attempt to deny what's happening to her, they retaliate, so as to crush her spirit even further (sealing her in a room) and tell her that her fate's been sealed (killing her).
The worlds are more reminders of her past. Most prominent are the hospitals, bringing to the forefront Sabitsuki's illness and a traumatic childhood of seeing many other children die around her. The school was where Sabitsuki used to hang out with her friends until she became too ill to go anymore. There are also lonely city streets, dark alleys, dingy apartment buildings, creepy overgrown gardens (possibly hospital courtyards, given the presence of sick children in some of them), bars, sewers, nightclubs, piers and the ocean, and cold snowy factory areas - all places in or near Sabitsuki's town.
The famicom world is based on her memories of video games she played as a child, and still plays to pass the time to this day.
The flesh-walled world is .flow showing Sabitsuki the state of her own body corrupted with germs (red demons) and with a weakened immune system (the black-haired girls dying off). The microscopic world is a look at the ill but strangely serene state of her cells as they succumb to the illness. Eventually, the red demons break out into other areas, diminishing the last of Sabitsuki's health.
The world known as Hell is .flow taunting Sabitsuki, saying that that's where she's going to go when she dies. The industrial maze and the rusted picture world are meant to be confusing and claustrophobic, as well as dangerous due to the plethora of hostile Kaibutsu, weakening Sabitsuki's resolve and instilling feelings of despair and fear.
The prison could represent a feeling of entrapment due to Sabitsuki's illness, perhaps meaning that her hospital stay felt more like a prison sentance; could be a reminder of some past crime (possibly committed with a handgun or steel pipe...) that she may or may not have been caught for; or it could be .flow telling Sabitsuki that she is, effectively, trapped and cannot escape no matter what.
The endgame is .flow taking over, influencing her to surrender all of her effects - everything that made her Sabitsuki - as she reaches the end of her life, thus giving up what little strength she had and causing her to lose sight of herself, simply becoming "Rust." .flow is only just waiting to unleash itself onto the real world to finish Sabitsuki/Rust off for good. It wants to wait until Sabitsuki/Rust is ready to go outside to look at the world one last time, delivering the crushing blow just before she's ready to let go for good.
The rooftop ending (keeping the effects) is Sabitsuki holding onto the last shreds of herself and leaving on her own terms before .flow can get to her.
The maid ending (surrendering the effects, but not finding the empty boxes) is .flow corrupting Sabitsuki, turning her into Rust and killing her off, possibly drawing her into itself via the maid to torment her in the afterlife.
The true ending (surrendering the effects and finding the empty boxes) is .flow corrupting Rust, but not before Rust manages to delve deep and discover the last hidden corners of her mind. She finds and kills off the corrupted part of her, gaining some control back. .flow is ready to kill her, but is not anticipating one last bit of ferocity on her part, as evidenced by the maid backing away in fear. Rust is attacked and killed, and put on display in the dream world, but manages to escape and ultimately remember herself as Sabitsuki.
Sabitsuki is an adult, and her room is a prison cell.
Having grown up in unsavory slums (hence all the urban areas), she was a lowly thug who "bit off more than she could chew" after a messed-up coup that unexpectedly ended in one, or perhaps multiple horrific murders (explaining the steel pipe and handgun — the Kaibutsu's faces immediately brought blunt trauma to this troper's mind). "Smile" may be an accomplice that betrayed her, or even a representation of her thug self. Some time during her bail, she contracts what is possibly a brain disease; it's taken a solid foothold and she's already fairly delirious by the time the game starts, and the Erosion Counter represents the stress of each event aggravating her condition. As for the computer? No idea.
Sabitsuki is some weird test tube baby.
And a unstable one at that
Sabitsuki had her family murdered in front of her
The melting girls event was a representation of that, with the big blue girl being Sabitsuki's mother, and the rest being her sisters.
There also, if I remember, is a hard-to-find world that is white, where abunch of blue girls stand around a dead blue girl.
The red demons and the Kaibutsu are competitive brothers and sisters
The Kaibutsu are, apparently, according to lol, officially female. The red demons don't seem to have a gender though, so let's assume they are male. The reason they replace each other is because they are fighting siblings, and they want to prove they can be better.
The cake in the Sugar Hole is made from human flesh
Which is why Gabe, the bar Kaibutsu, kills Sabitsuki and puts her in a "cupboard" type thing, which is actually an oven.
Sabitsuki's family was part of a cult
The melting girls event was actually some ritual her family did where they kidnapped some people and tortured them.
Smile and Sister's mother was one of the people who was tortured and killed by Sabitsuki's family rituals, which is why they act nervous around her, and Smile eventually kills her with a hammer
The room that neither Sabitsuki nor Rust can enter is something that was full of something horrifying, and so she cannot remember it.
This broom stick's magic was taught to her by the cult leaders.
Sabitsuki is a cosplayer
Her favorite thing to cosplay is Kiki from Kiki's Delivery Service.
Oreko is Sabitsuki's only friend
There is actually a friend meter, and certain specials are unlocked if Oreko is visited enough. Plus, Oreko trusts Sabitsuki enough to give her a diving helmet to help her come back.
Gabe isn't a Kaibutsu.
He doesn't appear to have a bleeding face like the other's, and he uses a weapon instead of sending people to the inescapable area. Also, instead of chasing people around, unless he is attacted, he is pretty calm and just seems to enjoy having drinks at the Sugar Hole.
He does get a bleeding face when he goes berserk.
Everything in Sabitsuki's .Flow world is a nightmare version of her real memories and friends and enemies
The Gas Mask Maid is a sick woman who has fun torturing children, and her alias she tells to the children is "Parade"
Which is why, when you go up the elevator, there is the word "Parade" written in blood, and the elevator leads to suffering children. It is also extremely near the Sugar Hole. Sabitsuki escaped from the "Parade house" when she was younger(Which is why there is a young Sabitsuki in a room), and so, for revenge for escaping, Parade/Gas Mask Maid saws Sabitsuki in half at the end.
The maid costume is a disguise, so is the gas mask. Her true form is Viscera, the woman from the melting girls event, and a door is found in one of the suffering children rooms that leads to her during the Endgame.
Sabitsuki is a Kaibutsu which hasn't been made hostile
Hostile Kaibutsu get more bloody faces, which can be pressumed that sane ones don't. Sabitsuki is perfectly sane in comparision, so she doesn't get bloodied... Except for parts at the end-game, either as Rust or in after the orange stairs where she is accompained by their laugh.
Although, the Kaibutsu's actually DO have a mild amount of blood on their faces, even the sane ones. Although, because of the Kaibutsu in the Corrupted School event, this could be true.
Smile represents Monoe
Monoe only unnervingly smiles, and Smile does too. And, Monoko is said to be Monoe's little sister by some, and Smile has his little Sister as well.
Sabitsuki's in a coma. Sister and the black-haired girl are the same person.
The entire game is a dream of Sabitsuki's as she lingers on life-support. This (sorta) explains why she uses her computer instead of her bed to access the dream world - because she's already asleep. The beginning and ending of the game, which both feature Sabitsuki floating in a black void, are meant to hint at this.
As for what put her in the coma, my theory is that Sabi used to be in Smile's gang, but decided to quit. In retaliation, several other members of the gang jumped her and beat her severely enough to leave her comatose. Sister, who was hanging out with Sabitsuki at the time, attempted to stand up to the attackers. Not knowing she was Smile's sister, they beat her up too. Whether or not Sister survived, I can't say, but she WAS horribly disfigured in the attack, which is why the black-haired girl's hair is always hanging in her face.
Sabitsuki blames herself for this. At the end of the game she sees Sister die over and over again, and then literally beats herself up over it.
The gas-mask maid killing Sabitsuki at the end is symbolic of her being taken off life support.
Sabitsuki is the evil side and has been killing everyone she knows.(Spoilers unmarked)
The reason Sabitsuki keeps getting blood in her room isn't from just a dream; it's all happening/all her memories. She gained a virus that drove her mad, creating a split personality, whom you play as for most of the game. Her vision of the world through memories is distorted and insane, and she kills and killed other people for her own enjoyment. She's had the disease for a long time, which is why she was in the hospital, and many of her victims were doctors and children, and the more strange the place, the older/less vivid the memory. Rust is the real person, and the end of the game shows her killing off her split personality after becoming tired of her sins. However, after symbolically killing off Sabitsuki, she still feels guilty for her crimes, and allows herself to be killed at the end of the game.
The Kaibutsu are kids who made fun of her ill condition. Their faces represent how she last saw them, with their faces bashed in.
The more frightening scenes where Sabitsuki is forced awake (usually by death) is the guilt her good side feels inside for murdering people.
The womb area represents how long she's been ill since birth/how another Sabitsuki has been "birthed" from within her.
Smile represents the only person who cared for Sabi, but also protected the kids from her when she went to school as the insane persona.
Oreko is the real Sabi, who hides herself from her other self until she kills her at the end of the game.
.flow is the Dark Woods Circus series.
I read on the Yume Nikki page that Madotsuki's dreams were Alice Human Sacrifice, so .flow is DWC.
Steel Cage Princess - Sabitsuki is sent to jail and has such a horrible experience she goes insane.
Wide Knowledge of the Late Madness - Sabitsuki is sent to a bad hospital where she is experimented on and deformed. ( Several effects suggest this.)
Dark Woods Circus - Deformed Sabi feels like she belongs in a circus, the song mentions a two-headed character, some people theorize Sabitsuki has a split personality. The deformed diva is her appearance, and the blue beast being how she feels; blue in this case meaning sad.
Blue Ice Castle - Sabitsuki goes insane again and kills everyone, including herself.
Red Swamp Bottom - Sabitsuki goes to Hell for her sins.
Guard and Scythe - Sabitsuki becomes a demon. ( Demon Sabi in the Oxidation ending.)
Absolutely everything in .flow is something to do with Sabitsuki's traumatic past and illness
I'm gonna attempt to make a 'Grand Unified Theory' on every effect and location, and significant NPC in .flow, going along with the popular 'Sabitsuki has an illness and her hallucinations/horrifying dreams are one of the results of it' theory. Plenty of unmarked spoilers.
Arm: An obvious homage to Yume Nikki's NPC, Monoko. Perhaps Yume Nikki is one of the games on Sabitsuki's computer? It'd explain why she's so psychologically messed up.
Black Hood: Given that this is found lying on a hospital bed in the Children's Hospital, perhaps this was a hoodie worn by a fellow patient who was in the same ward as Sabitsuki.
Broom: Sabitsuki may have liked to play Role Playing Games set in medieval periods before she lost her games and was only left with Shinsoku Neko, as this is found in the FC world. Perhaps her favorite character was a witch/mage of some sort.
Cat: This is the cat from Shinsoku Neko, the only game Sabitsuki has left to aide her boredom in her Quarantine room. She is so obsessed with her last game that she actually hallucinates/dreams about being the cat.
Corpse: In conjunction with the fact that one can access the morgue of the Children's Hospital, this effect suggests Sabitsuki has at some point seen a corpse. Perhaps she escaped from her hospital bed at some point and stumbled upon the morgue, and, horrified by what she had seen, was emotionally scarred.
Diving Helmet: Associated with Oreko, the only recurring NPC in the game that has an unseen 'friendship meter'. Perhaps Oreko was a friend of hers in the hospital ward who wore a large, clunky mask to prevent the spread of disease, and at some point, Sabitsuki was given one too, before newer measures were invented. The person remembered as Oreko would die of her illness before this new measure, so Sabitsuki never saw her face.
Dress: Perhaps Sabitsuki saw visitors of a person dying of a terminal illness going to see them for their last rites, or perhaps she has been to a funeral herself... perhaps one of her relatives had also died of the same illness Sabitsuki had... or perhaps she went to Oreko's funeral?
Gas Mask: Gas masks are a recurrent theme in .flow. The Gas Mask Maids wear them, and one of these ends up killing Sabitsuki in both the Maid Ending and the Golden Ending. Perhaps the Gas Masks are Sabitsuki's view on the 'new' medical masks (compared to the huge Diving Helmet), and any nurses which had to enter Sabitsuki's room to give her food would have to wear one of these.
Ghost: Sabitsuki might have believed in ghosts at some point, and perhaps this belief was cut off by the discovery of the Hospital Morgue, which is why this effect is found close to the nexus rather than deep within Sabitsuki's mind.
Handgun: At some point, Sabitsuki escaped the hospital she was in, and due to her years of isolation, went on a mad run throughout the city. She didn't go far though, heading to a nearby alley and hiding within a dumpster, she found a handgun, and when her pursuers approached her, she fired the gun into the air as a threat, but ultimately couldn't bring herself to shoot them, hence why she can't do so in .flow either. This led her to get caught an put under house arrest/quarantine along with life support machines.
Headphones: Just about her only source of entertainment in the hospital, and possibly the associated music player had psychadelic visualizations, as you find the headphones in the Neon World, and the only other music-related area is the Rave Maze/Psychellic streets, an equally bright and colorful area.
Iron Pipe: Perhaps one of the weapons Sabitsuki acquired when she escaped the hospital, and unlike the handgun, she could bring herself to use it, and attacked people, making them look like Kaibutsu. Alternatively, perhaps a hospital massacre happened (the Corrupted Children's Hospital event) and one of the perpetrators attacked the children with an iron pipe, and Sabitsuki was lucky enough to not be noticed, hence why there are several dying and bloodied child Kaibutsu, but the child form of Sabitsuki is sitting up her bed. Notably, anyone that dies via the Iron Pipe makes a child-like scream.
Machine: Perhaps cybernetic implants were an experimental cure for Sabitsuki's disease. If so, this might be her view on it.
Mono-eye: This is a warped memory of when she saw a fellow patient in the later stages of her disease (which is what the lunatic Kaibutsu/decayed Rust is based on) when she was still young, and thus was unable to comprehend what she saw, so she interpreted it as a single eye.
Plant: Perhaps, like the machine, plant grafts were one of the experimental procedures that would act as a cure to Sabitsuki's illness. Evidence for this is more compelling- the past version of the plant labyrinth is accessible by the Children's Hospital, and the first room accessible from there is full of child Kaibutsu as well as Little Sabitsuki with plant grafts similar in look to the plant effect.
Psychedelic: Most likely an embodiment of the psychedelic visuals of Sabitsuki's music player, as mentioned in the Headphone effect explanation.
Slime: Perhaps Sabitsuki witnessed other forms of terminal illness in her ward, including someone who was coughing up mucus, or slime. It'd also explain why the only other significant experience with slime is accessing the horrifying Bloody Room event, which forces Sabitsuki awake, suggesting perhaps she saw this same person in the morgue.
Tattoo: Notably, you get this from an oversized form of Smile, who has similar eye markings to the effect in his normal form. Perhaps Smile was some kind of doctor who had cross tattoos around his eyes from when he was younger (and stupider), and thus, Sabitsuki associated Tattoos on the eye with him. Alternatively, if Sabitsuki ever got to attend school, Smile was a hippy teacher, but was otherwise the same with regards to the Tattoo effect.
Television: The Television which Sabitsuki uses to play her game console, hence why it's in the FC World.
Tumbling Doll: Perhaps, in conjunction with the Iron Pipe theory, one of the Hospital Massacre perpetrators wielded a Chainsaw. Perhaps this particular perpetrator was supposedly a Nurse, but was actually the one who had planned the massacre, and thus wore a gas-mask like all the other nurses. She would also be the one that Sabitsuki remembers and gets killed by in two of the endings.
Uniform: Presumably Sabitsuki attended school at some point, but when in her childhood is anyone's guess. That's probably the single biggest hole in this theory... where did her school-time go?
Viscera: Probably one of the many traumatic sights witnessed at her hospital's morgue.
Watering Can: Given the overall disturbing depiction of Plants in .flow, it can be assumed Sabitsuki had seen plants being cultured and watered to be prepared for the presumably horrific plant grafts.
Whistle: In whatever short term at school Sabitsuki had, she had to do gym class. The result was that the teacher used a whistle, at which point, she'd run. She began to associate subconsiously, a la Pavlov's Dog, whistling with movement.
The Three Empty Boxes
Dying Girls Event Reprise: This area is almost the same as the normal 'Dying Girls Event', however instead of having to kill the girls to get to the room with the empty box, the girls crumble apart in bloody piles of their own accord, like that of the final room in the original Dying Girls event. I'm going to go with a popular theory that the black-haired girls represent Sabitsuki's immune system, as they are only found in large amounts in the same area the Red Demons (which slowly degenerate 'Rust', meaning they could be representative of Sabitsuki's disease). If that is the case, then this event is showing that Sabitsuki/Rust's disease is progressing into its final stages, and instead of her having to destroy parts of her immune system to see a large part of her immune system die on its own, the disease has gone far enough that it can do this all on its own.
Oreko's Secret Lab/Test-tube Sabitsuki: It seems that Rust is visiting the most traumatic events in the game, the ones which regular Sabitsuki could not. In this case, perhaps the Children's Hospital was indeed something far more sinister than just that, and were so obsessed with finding a cure to Sabitsuki's (and presumably, the other children seen as Kaibutsu in .flow) disease that they grew clones to test cures. This would explain why a plant-grafted Child Sabitsuki can be seen, yet real-life Sabitsuki doesn't have plant grafts herself. Sabitsuki perhaps saw a clone being matured and was traumatised from the event... and perhaps Oreko was one of these clones, hence why she's associated with this particular empty box and is also found in a test tube.
The Mouth in the School Basement: A recurring theme in .flow is the presence of mouths. One of the worlds directly accessible from the nexus has plenty of disembodied, bloody mouths, occasionally your dream TV will show a mouth (along with a Kaibutsu's laughter), the Flesh Walls world allows access to an area with Red Demons that has a background that is effectively a massive, frightening, bloody maw of a mouth. Of course, both the School and the Corrupted School are famous for having plenty of pictures of mouths all over the walls. Bloodied mouths seem to be a symptom of Sabitsuki's disease, and this would explain why blood appears on the keyboard of your computer after you experience a traumatic event in .flow... this is Sabitsuki coughing from the mouth. So thus, to top it all off, in the endgame, of of the empty boxes comes from the exact same thing- a giant gaping maw on the wall of the school basement.
Areas of .flow
Alleyway/Children's Hospital: The Alleyway Hospital is one of the most disturbing areas in the game, and contains the infamous 'Parade' corridor, which is filled with suffering or dying children. This 'Parade' could refer to the previously mentioned WMG that says that Parade is a Gas Mask Maid which wanted to hurt the children, leading to the Iron-pipe and Chainsaw massacre that leads to the sights Sabitsuki witnessed in the 'Parade' corridor. Most notably, 'Little Sabitsuki' is bed-ridden, but ultimately unhurt by the massacre in this corridor. This area would later lead to an Empty Box, so clearly, this is a significant trauma to Sabitsuki.
Apartments: Perhaps at some point, Sabitsuki lived in one? Or perhaps this is what the lower levels of Sabitsuki's current building of residence looks like...
Bone Maze: This area, although small, is highly unnerving, and is filled with blood spatters, chunks of bone, and noticeably, a large gaping mouth with blood dripping from it, maintaining what seems to be a recurrent theme. Suggests, along with many other places, that Sabitsuki's disease has its most noticeable symptoms near her mouth.
City Coast/Outskirts/Psychedelic City: This is an area in which one can access the Rave Maze, and the sewers, and so, in a sense, is organised like a coherent city. Perhaps the fact it connects to the rave maze suggests Sabitsuki walked through a city with her headphones on one time, and it was psychedelic in her memory because she associates music with the psychedelic visualizations of her music player. In addition, this theory is supported by the fact what seems to be a rave club is in this city, associating it further with music.
Dark Corridors: This area may have been reminiscent of a time she went out into the aforementioned city in the dark, or tried to escape the Hospital she was in at night, although that memory would most likely be associated with the Hospital corridor.
Decaying Art Gallery: The decaying art gallery is an area which, similarly to the sewers of Yume Nikki, has windows that allow the player to see unsettling, scrawled images. One of these images looks surprisingly close to the background of the Red Body world, so perhaps that particularly disturbing image, along with all the others in the art gallery, was drawn by Sabitsuki as she began to go insane.
FC World: Sabitsuki possibly owned more games than just Shinsoku Neko, and perhaps even played Mother 1 and other, more traditional RPGs. If so, it would explain this area, and why both the Witch (typical player type in classic RPGs) and Television (as in, what connects to a games console) effects are found here.
Flesh Walls World: The most common, and in my opinion, leading theory here is that this area is representative of the diseased parts of Sabitsuki's body and mind, as its the only place that contains the Red Demons until the Rust Chapter, and going along with the 'Black haired girls being her immune system' theory, this also explains why the black haired girls are, for the most part, only found here too, most likely fighting the infection. The Dying Girls Event, found in a unique area of this part, is representative of the disease getting a greater and greater advantage over Sabitsuki's body.
Footprint Path: This could perhaps represent a time when Sabitsuki could walk without pain, as it leads to the Rave Maze, where she can go to the Psychedelic City (and I have already mentioned my theories regarding this). However, it also leads to the Plant Labyrinth, a surprisingly disturbing area which can lead to the horrifying Children's Hospital and the White Room, both of which contain ill, dying, or dead Child Kaibutsu as well as a plant-grafted Child Sabitsuki, which may mean she associates her walking with whatever led her to end up contracting the disease she had.
Geometry/Computer-grid World: Unknown, perhaps associated with the displays of her hospital monitors, as to access it, you have to look through one underwater.
Ghost Garden: Possibly representative of Sabitsuki's fear of death, given this is where she finds the Ghost effect. Perhaps representative of her hopes for an afterlife.
Hell: The dark side of Sabitsuki's belief in the afterlife. She has gone insane due to her disease, and attempted to escape the hospital and attempting to fend off her pursuers with a handgun (see handgun effect theory), and she perhaps sees herself as too evil to get into a blissful afterlife/become a ghost, so she thinks she could go to hell, after being executed for her crimes (the world is accessed by being Guillotined.)
Hospital Corridors: This could have been an attempt to escape the hospital, but as the corridors all looked the same, she had a confusing time finding a way out.
Industrial Maze: This could be representative of another of Sabitsuki's walks in the city, and how confusing it was. Perhaps this time round, she didn't have her headphones, and so was more wary of how lost she was and didn't associate the memory with a Psychedelic feel.
Industrial Snow World: Unknown, but the surprisingly disturbing Plant Labyrinth is access from here by triggering a plant that causes the entire area to be overgrown with plants. Perhaps this is particularly disturbing to Sabitsuki, and is one of her fears that plants will over-run everything, due to her experiences with the Plant Grafted children.
Isometric Pathway: A transitional area that most likely represents being confused before finding home, as all exits of the maze either lead to the FC World or the Apartments.
Misty Monochrome World: This area is monochromatic and has what seem to be bacteria-like organisms floating about, which change the layout of the level, causing it to either gain or lose structure. The whole area looks like an image from a Transmission Electron Microscope, a microscope used to examine smaller cells like Bacteria extremely closely, but can only produce non-colour, 2D images. Perhaps Sabitsuki had seen images from her diagnosis when she contracted her illness, and they looked like this world.
Monochrome World: A tiny world. Significance unknown, perhaps similar to the scrawlings of the Decaying Art Gallery.
The Nexus: A shattered, four-way road, representing the shattered, dissociated structure of Sabitsuki's mind.
Neon World: An area that represents Sabitsuki's love of music, as it's where she finds the Headphones effect and is filled with Psychedelic versions of walking instruments and notes. Notably, the only other Psychedelic places in the game have a similar association with music, either the Rave Maze or the Psychedelic City (which contains a club).
Orange Stairway: This may be representative of the stairs Sabitsuki used to climb to visit fellow patient Oreko while in the hospital ward (see Diving Helmet). She used an orange light on the staircase as an indicator of whether she had walked too far up the stairs or not.
Overgrown Islands: This area is only accessed by a secondary Ghost Garden, one which is accessed by the Plant Labyrinth. Perhaps this area is representative of the many Sabitsuki Clones (see the Empty Boxes section) that died during the Plant Graft attempts to cure patients of the disease.
Pipe World: This area is right next to the Nexus, and is where the Iron Pipe is found. Perhaps the Child Hospital Massacre is fairly easy for Sabitsuki to recall, or, at least, one of the weapons used, but she suppressed the truly gruesome details, which have to be found by going to the Children's Hospital of Sabitsuki's memories.
Plant Labyrinth (Past and Present): There are two versions of the Plant Labyrinth, and both have surprisingly disturbing music despite the fact that, all in all, it is simply an overground, ruined area... until one reaches the room that leads to the Children's Hospital and the White Room, which is filled with dying, ill, or dead children, the former having Plant Grafts all over their bodies. This suggests that Sabitsuki associates plants with something highly traumatic, such as experimental attempts to cure her disease.
Prison: Representative of Sabitsuki's guilt over her homicidal defense of herself when she attempted to escape the hospital.
Rave Maze: Most of this has been discussed in the Neon World and Psychedelic City theories, but this could be representative of Sabitsuki's love of music, perhaps walking with her headphones on.
Red Eye World: A world which leads to the traumatic Corrupted School event by chance and is adorned with feet, eyes, and bloodied mouths. This is one of the many areas which support the disease theory, and it suggests a bloodied mouth is the most common symptom of the disease. The fact that this area leads to the corrupted school suggests Sabitsuki either gave the disease to many fellow class members or was bullied for her disease. If it was the former, this would no doubt have added to her guilt.
Red Body World: An enlarged version of one of Sabitsuki's many scrawlings found in the Decaying Art Gallery.
Rust Corridor: The final corridor in the game, assuming you're going for the Golden Ending. This is the source of the disease, the most infected part of her body. Rust finally goes into this area, and finds the area that is allowing the disease to propagate and survive (perhaps a burst tubercle in her lung, for example) and destroys the cause (represented by a Demonic version of Sabitsuki). This causes the disease to be finally cured, but unfortunately, this last immune response backfired enough to make it fatal for Sabitsuki/Rust as well as her having irreparable psychological damage, causing her to hallucinate being killed in her last minutes of her life.
Rusty Pier/Slums: The area where one can access the Slums and school. This suggests Sabitsuki lived near the coast as a child, in a poor area. Perhaps this is why she was diseased- she was not in a hygienic environment.
School/Corrupted School: The School is a area full of schoolchildren, Passive Kaibutsu, and posters of bloodied mouths, as well as being home to Smile (who is also in the slums). Given this is an Area where one finds an Empty Box as Rust, it can be fair to say something traumatic occurred here, like a set of bullies attacking Sabitsuki, or Sabitsuki going to school one day to find every child having a similar disease to herself, hence why bloodied mouths are prevalent in the area. The children may have blamed Sabitsuki, or Sabitsuki may have blamed herself, and began to think her schoolmates hated her (hence why in the Corrupt School Event, accessed by a Bloodied Mouth) she is chased by Kaibutsu wielding Iron Pipes, who wear a similar uniform to her Uniform Effect.
Sewers: Most likely just her opinion of what Sewers look like. That said, the massive Slime Pile in this area leads to a White Room that looks like it is from a hospital covered in blood, which may suggest the fellow patient who excessively produced Mucus (see Slime Effect) was one of the many victims brutally murdered in the 'Parade Massacre', and Sabitsuki actually managed to see the dead body, but repressed the memory, hence why you can't access the room behind the door in the blood-covered room.
Starry Pier: Unknown, perhaps one of Sabitsuki's few pleasant memories of stargazing.
The Sugar Hole: This area notably has the Gas-mask maids in it. All of them are passive, however, unlike the Chainsaw-wielding one at the end, and this suggests it is representative of the area in the Hospital Sabitsuki went to to get food and drink from the nurses (see Gas Mask Effect) that weren't psychotic.
The Sugar Hole Painting: This area is representative of the office of 'Parade', the psycho Nurse who organised the Children's Hospital Massacre, as the Chainsaw Maid is present, her Chainsaw next to her, allowing you to get the Tumbling Doll effect.
Underwater: Possibly representative of Sabitsuki's view of things with her clunky, old quarantine mask (see Diving Helmet), as this is where one can find both Oreko and the Diving Helmet effect, and there are hospital monitors in the area.
Underwater Temple: Unknown, perhaps once again representative of the confusing Hospital Corridors, this time remembered from the view of Sabitsuki's 'old' mask...
White Maze: The area required to access the Misty Microscopic World, and most likely a representative of a Hospital Corridor Sabitsuki walked down to access to TEM images of her diagnostic results if it is to fit in with this WMG's current theory.
White World: An unsettling area filled with ill children who are all walking in the same direction, aside from a few which are stopping to look at a dead black-haired girl. Possibly representative of the combined hell all sufferers of the disease are going through, and their mourning of the times they were healthy (the black-haired, 'immune system' girl).
The Black-haired Girls: The Black-Haired Girls seem to represent Sabitsuki's immune system. After witnessing an event which shows a large amount of them dying out of their own accord, Sabitsuki's room will increase somewhat in how covered in blood it is, showing that the illness has started to progress. Also, when playing as Rust (Sabitsuki who is fairly close to succumbing to the illness), one of the Empty Boxes is accessed by going into a room filled with the Girls dying off piece by piece. The fact that, with the exception of one lying dead in the White Room, all of them are found in the flesh walls world, the same world as the Red Demons, indicates they are at least somewhat related, and their mass death being in conjunction with the Red Demons' increase suggests they are at least opposed to them in some way.
Demon Sabitsuki: My theory with this is that it is the core of Sabitsuki's illness, the primary infection area from which Sabitsuki's illness propagates itself from. Perhaps the illness is similar to Tuberculosis (a respiratory illness), as Sabitsuki seems to cough up blood, or at least, have blood in the mouth due to the illness. If it is like tuberculosis, then it also infects in stages. The first stage is a primary infection, which may cause mild symptoms in most people but may affect younger and older individuals more (hence why Child Sabitsuki is bedridden). The immune system deals with this primary infection by sealing the infection in a 'Tubercle' (hence the name) that seals the bacteria within it, and is a sort of fleshy lump. While the illness was latant, Sabitsuki attended school. Later, however, the secondary infection began. This is where the Tubercle is compromised and the bacteria contained within it escape. This is the phase Sabitsuki is currently in. Thus, Demon Sabitsuki is this ruptured tubercle, and Rust acts as a replacement for her destroyed immune system and rips her way through her lungs, destroying the very source of the illness (this is actually why people die of tuberculosis- the immune system damages the lung tissue as collateral damage fighting the Bacterial infection, causing sufferers to cough up blood and die) but also leaving herself dying in the process (hence why it an image of herself that she has to kill). She staggers outside, and suffers one last hallucination, first of a Gas Mask Maid, and secondly of herself going on one last pleasant headphone stroll.
The Gas-mask Maids: These may have been nurses who served Sabitsuki, wearing Quarantine masks, as they serve food, and in general are polite to Sabitsuki. However, the one at the end wields a chainsaw, and there is one standing next to the Chainsaw from which you acquire the Tumbling Doll Effect. This suggests that at least one of the people remembered by the delusional Sabitsuki as a Gas Mask Maid was also a psychopath, a killer, and, as my earlier parts of this WMG mentioned, was the organizer of the Hospital Massacre which is remembered by Sabitsuki in the 'Parade' Corridor.
The Kaibutsu: These were most likely fellow sufferers of Sabitsuki's disease. Evidence for this comes from the fact that where there are Child Kaibutsu in the Children's Hospital and Plant Labyrinth, there is also Little Sabitsuki, not to mention one of the full-grown Kaibutsu resembles Sabitsuki a little. And all adult ones wear School Uniforms and have white hair, suggesting Sabitsuki not only infected fellow classmates, but that Albinism, or at least white hair, is a symptom of the illness. The fact that some are hostile represents Sabitsuki's guit and belief that they hate her for infecting them.
Little/Child Sabitsuki: This is Sabitsuki remembering her past as a patient in the Children's hospital. Only found in the Corrupted Children's Hospital and Plant Labyrinth, it can be assumed many traumatic events occurred in this period of her childhood, and perhaps she deteriorated significantly, as she is also an indicator of the 'Erosion Meter', similarly to the blood stains in the Real World. Depending on how 'Eroded' Sabitsuki is, Little Sabitsuki can be healthy, ill, or terminally ill, each phase being progressively more bloody in the face. Given the Kaibutsu are the same, perhaps the illness attacks both the lungs and the mind, or perhaps it is an infection of the head in general, including the mouth, causing the coughing of blood and bloody mouth motifs.
Monochrome Machine Girl: While a notably unique NPC, there isn't much to say, beyond the girl has similar mechanical legs to the 'Machine' effect, meaning that perhaps this is a highly vague memory of a fellow patient who was undertaking the experimental cybernetic attempt at a cure to the disease?
Oreko: Oreko is a recurring NPC that shows up in a few places. Firstly, Underwater (an Area connected to the hospital), and from then on, in her own house and in her secret lab, holding hands with a battered Sabitsuki (although this is only if your friendship level is high enough with her). This suggests that Sabitsuki saw whoever is being represented by Oreko as a friend, and is able to put on a Diving Helmet similar to her own. As mentioned in the Diving Helmet section, Oreko could be a fellow patient who was a friend during an era when the hospital used large, clunky Quarantine masks to cover patients' faces. Given that a test-tube version of Sabitsuki is found in Oreko's Lab as Rust, perhaps this could mean that Oreko is in fact a clone of Sabitsuki herself, one of the clones used in experimental cures like the Machine and Plant Effect. She would have died before the smaller, sleeker 'Gas Mask' medical masks were introduced, so Sabitsuki associates her and her 'Diving Helmet' with a time when she was happy and had friends.
Red Demons: Here it is. The big one. Red Demons are representative of the pathogens that are wreaking havoc throughout Sabitsuki/Rust's body. Although at first, they are under control, and are only within the Flesh Walls World, and are mostly harmless (like a contained primary infection), once the disease progresses and the secondary infection stage occurs, the tubercle or containment is ruptured and they soon infect her body to a far greater extent. Given this illness is more than just Tuberculosis and has psychological as well as physical symptoms, this illness, while already (relatively) mildly screwing with her brain, once the secondary infection stage occurs her mind becomes all the more consumed by the illness, with memories being out and out replaced by the Red Demons (hence why Kaibutsu are no longer present/become assimilated in Rust's version of the Corrupted School Event). This culminates with Rust having to get to the bottom of this and attacking the disease at it's core- the ruptured Tubercle/containment, which is the Rust Corridor, where the highest density of Red Demons are in the Rust Chapter. There, she destroys (sterilizes) the area, which amounts to killing Demon Sabitsuki, and rids herself of the Red Demons, but not without significantly harming herself in the process.
Smile/Sister: Smile's role is unknown, perhaps a friend/family member/teacher/doctor which she knew. The fact that in the Corrupted School Event you have no choice to kill him must have meant Sabitsuki attacked him for some reason out of what she considered to be necessity. Perhaps when Sabitsuki infected her classmates, Smile was a teacher who noticed this and knew enoough about Sabitsuki to report her. Sabitsuki didn't want it being reported, as it would reveal she was the one responsible, and so she attacked Smile, reasoning with her twisted logic that she had to, and Smile, having no choice, defended himself and knocked Sabitsuki out (why she blacks out). Perhaps before this, she and Smile got along, as she seems to like him enough to want his eye tattoos (the Tattoo effect, got from interacting with a large Smile on the rooftop). 'Sister', the girl hiding behind Smile in his house in the Slums, may indicate that Smile had a daughter or other female dependent like a housewife or sister, and Sabitsuki may have even tried to attack her, hence why she hides behind Smile constantly.
Well, that's it for now. Be sure to add to it if you can think of something I've missed.
The reason why she won't go out? Because she can't risk infecting others.
The game frequently implies that Sabitsuki and the Kaibutsu are similar; both have white hair, the Kaibutsu wear Sabi's uniform, Sabi can be seen with a Slasher Smile at several points, and the Kaibutsu laugh is heard multiple times outside of actually interacting with Kaibutsu. The symptoms of zombie infection apparently include white hair, Blood from the Mouth and laughing at inappropriate moments, as well as being infected through bites (explaining all the mouth imagery). The Corrupted School event could be indicative of her classmates turning. However, the reason why the younger Sabitsuki also has white hair has yet to be explained.
Smile could be the one who infected her. He was someone who went to her school, and was a Bully Hunter. Sabi had a reputation for being a Delinquent, which Smile would always stand up against when she bothered her classmates. Eventually, however, the two became Vitriolic Best Buds, even coming over to his house to hang with him and his sister. At one point, Smile was bitten as well, and tried to hide it, such as dying his hair black. Sabi realized that he was slowly turning into a "Kaibutsu", and attempted a Mercy Kill in the basement of the school with an iron pipe, only to be attacked by him in the process. Perhaps he was lucid enough at this point to use a crowbar, but not enough to not bite her. The markings around his eyes are not tattoos, but in some way connected to his infection.
The Black Haired Girls represent, again, her immune system, while the Red Devils are The Virus rampaging through her system. As the Devils infect her, they slowly peel away at her humanity, first by taking away her ability to use items, then slowly turning her into one of them.
Many of the items allude to this. For example, the Viscera could be a point where she saw someone literally being ripped apart by the Kaibutsu. The corpse is what the Kaibutsu literally are; walking corpses. Tumbling doll and the machine could indicate that she was bitten on a limb, and amputation was considered to prevent her infection from spreading. The dress could indicate mourning, the ghost is what she wishes she would turn into instead, and the arm is a distortion of a dead body that she once saw (either that, or as noted in a previous WMG she simply likes Yume Nikki due to its Shout-Out nature). Mono Eye could be another distortion of an event involving Kaibutsu that she firsthand witnessed. Many of the other effects, though, simply allude to her background or things she liked before being infected; for example, the hood and the gun are memories of her being a Delinquent/growing up in a bad neighborhood, while the plant suggests she liked gardens, the cat, alongside the game and pillow, reveals she was a "cat person", the broom might suggest that she liked Kiki's Delivery Service as a child, her headphones being in the dreams/hallucinations due to Sabi enjoying music, and so on.
Oreko was a childhood friend of hers, that enjoyed the ocean and wanted to work on machines. She still tries to see her and comfort her in her last moments, but if Sabi attacks her in a fit caused by her infection she disappears, having left. The helmet is a distortion of both the gas mask and the fact that she was looking into being an engineer for ocean technology.
Many of the areas are memories of her daily life. The Sugar Hole was a restaurant she used to go to, while there may have been an art gallery nearby. Further proof of her living in a bad neighborhood are the various references to city areas that look very grimy. Flesh Walls is, once again, her body.
The "Rust" segment is when Sabitsuki's condition progresses further, and she loses her humanity more. In this stage, she remembers how she got infected, hallucinates seeing what's happening in her body (same with Dying Girls), and realizes that she will lose her identity to The Virus. The Rusted Corridor is her hallucinating about killing the virus at its source, fantasized as a demonic version of herself.
In the ending, she completely turns. The Gas Mask Maids were her delusional interpretation of her caretakers during this period; they wear gas masks to avoid infection and use chainsaws to attack the Kaibutsu. The hospital imagery is because initially they thought the virus could be treated, and while they manage to slow down its effect on Sabi, it didn't destroy it completely (possibly she was a test subject in an experiment?). The only option is to kill her.
I like this a lot. It fits in well with my theory (the huge-ass one above yours), and adds a few more symptoms as well as explaining Sabitsuki's apparent insanity.
.Flow is the Dying Dream of a girl with a terminal illness.
Sabitsuki isn't actually sick: she's just having her period.
More specifically, she's experiencing her very first one, or has a very irregular cycle. Either way, Sabitsuki's not used to menstruation. This manifests in blood everywhere in her dreams and everything messed up in the game, since that's how she envisions the whole concept.
She only thinks she's sick, but really, she just doesn't know what to do in that situation. The blood in real life could be she's flowing unusually heavy, so it ends up in her room with no way to clean it up.
So Sabitsuki isn't really sick, just really scared and confused at the concept of becoming a woman.