I view her as someone with a fascination with strange dark things. She doesn't care about the rules and does what she wants.
I think of her as not worrying about what others think of her because she's too busy struggling to give it all to what she thinks she should. Either keeping a mental stability or looking further into whatever .flow is.
Not selfish, but not someone who normally approaches others. She does get really lonely, and this loneliness at the game's point is something she had accepted already as hopeless.
Oreko is probably the closest to a good friend she ever had, because she acted friendly to her. (Taking 0.16 as actual events, anyways)
She generally has a strong self, but she's been thorougly broken. I'd say she's solitary.
While she doesn't care about the grotesque or the weird stuff, and is pretty cold, she does genuinely fear and is tormented and sad. It's because of how much pain she's in she focuses the most she can on doing what she thinks she must.
As for Rust… I think at that point of the game Sabitsuki loses herself and leaves a mostly empty husk. It doesn't feel anymore, no torment, no anything.
It is a thing that only follows its "programming" of clearing up the traumatic events of Sabitsuki, that when she was a person was too vulnerable to whistand.
Just an empty husk, that of course, gets filled with the corruption which ultimately takes over, and kills the last that remained of Sabitsuki in the end.
Appears perfectly normal to most people. Actually, has a very intense, obsessive personality. Generally morbid/macabre and violent. Prone to intense moodswings, and very temperamental, likely a sociopath. Quiet and solitary, but incredibly observant. Doesn't make friends easily, and makes hardly any effort to. Despite this, if other people pique her interest enough, she will obsess over them until they like her back, and will be willing to fight to the death for them. In such relationships, she is surprisingly caring. Fiercely loyal, to both her morals, and the people she likes. Zodiac-wise, most likely a Scorpio (no offence, Scorpios!!).
I always imagined Sabitsuki as observant and intense, but shy. Through her self-isolation she has developed an obsession for those things on the edges of what is considered "human", from the colorful and whimsical (like imitating hallucinations, almost) to the dark and macabre (idealizing blood and gore). Sabitsuki tends to avoid rather than deal with her problems (gun effect makes people go away, whistle affect makes people go where she wants, the only thing that really interacts is the pipe, but that's just making them "fade away")
Rust, on the other hand, is just like Sabitsuki, except Rust refuses to face away from any obstacle. Rust's intensity is a grim acceptance of reality for the horrors it is. This isn't so much psychopathy as much as it is a single-minded determination, regardless of injury to herself or others. I believe that Rust is an aspect of Sabitsuki's need to be sheltered and protected. Rust is the one that heads into danger, not Sabitsuki. Rust is the one that faces the monsters and gets hurt, not Sabitsuki. Rust is Sabitsuki's excuse for the potentially psychopathic things she does as a reaction to trauma or perceived dangers. Rust does what Sabitsuki is afraid to do, and so Rust is what Sabitsuki is afraid of being, but needs to be when she feels pushes to her limit of her avoidance.