Relatives everywhere
Metal-tan also known as machine girl can be lost older sister of Smile and Hou (Smile's lil sis), this can be proved by fact that she have same cross like tattoos on her eyes just like Smile.
Theory 1/example
Based on how Sabitsuki never leaves her room unless she accesses her computer, she may be known as a shut in. some of the NPCs she encounters may be some of the only friends she is able to have or access. Hating reality, she goes onto her computer and tries to escape responsibilities in her life. In the School, she gets chased by the Red Demons, or Kaibutsus, at a certain point. This could mean she was bullied or even harassed. While you see countless demons trying to get her while she is running through the hallway, this could mean she is a target in her school. Knowing how Smile never seems to be a nuisance by trying to kill her in anyway, could be a friend to her as well as some of the other NPCs. However, you see Smile the most around the dream world, possibly for support. Another example is how she can get possible revenge on a certain NPC Kaibutsu. How she tries to stand up to him when you select the Pipe effect. When you try to hit it, it automatically turns around and kills Sabi
Another theory I have came up with is that .flow takes place in an apocalyptic world. Seeing how it is always a darkish grey green sky when you go upon the balcony. The Human Brain is also not creative enough to imagine another living human being. Possibly based how Sabitsuki can go outside on her balcony that is the only possible way for her to see any other human. As for smile and sister, they could possibly be neighbors or someone who she just happened to see all the time, possibly a close friend maybe.
Sabitsuki is diseased with "Rust"
The most telling sign for this theory is that Sabitsuki's name means "covered in rust".
Sabitsuki may or may not be able to leave her room due to being quarantined. The disease, known as "rust", has visual signs of infection - namely, the rusty coloured skin/potentially blood discolouring many characters skin. There are lots of dead and dying children in the corrupted Children's Hospital, these children (especially due to Little Sabitsuki in her bed there) could very well have also suffered from the rust disease. Part of the disease could also be present in the Erosion Counter - certain traumatising events cause Sabitsuki to be forced out of .flow and into the real world, with blood splatters (and later, weird machine-like things hooked to her monitor) appearing in her room. This could be a start to her body succumbing to the rust.
Red Demons are the disease attacking Sabitsuki
There are almost no demons when playing as Sabitsuki, the only ones appearing in the Flesh Walls world, which is heavily theorised to represent Sabitsuki's body. If this is assumed to be true, the demons in the Flesh Walls world could represent the rust starting its take over. Once dropping the effects into the PC, a small clip shows a small Sabitsuki being birthed in a mass of viscera. This could represent the rust's full takeover of Sabitsuki, hence the reason Sabitsuki from then on is referred to as Rust, and her colour slightly changes to a more sickly tint. Ending 3, also known as the True Ending, shows Rust walking through long halls that highly resemble the Children's Hospital. Each hall gets more and more deteriorated as more demons start attacking her. Rust reaches another version of herself (or Sabitsuki) in a throne and kills her. This could signify Rust taking over and finally killing Sabitsuki, or Sabitsuki killing the rust disease inside of her, depending upon your interpretation.
The Maid in the endings is fairly innocent
The other two endings, instead of revealing less of the final true ending, actually show exactly what happened. The first ending is due to Rust not having control of Sabitsuki - she finally leaves her room. In the second ending, Rust begins taking over Sabitsuki's body but hasn't fully killed her yet (gone through the corridor). The Sugar Maid comes in to take Sabitsuki away to take care of her, and Sabitsuki ends up dying in her arms. The third ending, Rust has fully taken over Sabitsuki's body and the Sugar Maid comes in and kills her to rid her of the disease once and for all (or possibly in self defence, as Rust comes closer to her with a grin on her face).
The black-haired girls are Sabitsuki's immune system
The girls who heavily appear in the Flesh Walls world could easily represent Sabitsuki's immune system. The ones outside of the Flesh Walls area appear to be sick or dead - the one in the Hospital is in a bed, and the one in the white room with the dead children. To get to the Dying Girls event, you may have to kill them off to see the scene (however, this can be avoided with the use of the whistle effect). Going near any of the girls as Rust will automatically kill them. When getting the hospital GET as Rust, once entering the room preceding the Dying Girls event, every black haired girl in the room dies. This could symbolise Rust and her disease brutalising Sabitsuki's body and her immune system.