Rust is heavily implied to be Sabitsuki succumbing to the influence of the kaibutsu, given that the physical damage she takes from them (as Rust cannot use effects) gives her a similar appearance to the red demons, such as horns and blood across her body (blood on her face/bloody clothing) which is supposed to represent actual rust spreading across her. The endings of the game show that Sabitsuki becoming Rust winds up making her an enemy of the Cleaners who (apparently) exist in the real world.