Sabitsuki (錆つき) means "rust-covered" or "rust-eaten".
Protagonist of .flow.
Sabitsuki (さびつき) is the main protagonist of .flow.
Sabitsuki is a girl with short white messy hair. Her outfit consists of a dusty purple tank top, white skirt, and dark red shoes. Equipping effects while in flow will slightly alter her appearance, but her general design never changes.
Like Madotsuki, her eyes appear to be always closed in-game (with the exception of her Eye and possibly Cat effects). Artwork from lolrust, however, commonly depicts her with red eyes, and lol has officially said that her eyes are "a light brown with a reddish tinge." She has short white hair in a lopsided bed-head style.
Rust is the deuteragonist of .flow, serving as the main playable character after certain criteria are met.
After collecting all 24 Effects, Sabitsuki can, while already in flow, access and use her computer again. This allows her to discard all of her Effects, transforming her into Rust for the remainder of the game, and affecting much of her dream world.
Rust appears to be an alter-ego of sorts who largely looks identical to Sabitsuki herself. In fact, while awake, she looks completely identical to her. However, while in flow, she appears very slightly different: Her hair is slightly paler, her skin has an unhealthy, almost olive pallor, and her clothes are slightly more detailed, with wrinkles now visible on both her tank top and skirt.
Unlike Sabitsuki, who was either trapped in inescapable areas or had her Effect forcibly removed if caught by Chasers, Rust will instead take damage if caught by the Fetuses, which replace all Chasers after discarding the Effects. Her appearance will noticeably decay the more damage she takes, rapidly growing bloodier as she nears death. Rust effectively has 15 "hit points" - if she is caught by Fetuses 15 times in one flow session, she will die, her head splitting open as she collapses into a pile of gore. Dying in this manner will wake her up, forcing her to reenter flow and resume her investigation from the Entrance.
Rust cannot use any Effect whatsoever, leaving her with very few options for evading Fetuses or navigating her dream world. However, in exchange, she can enter certain locations that Sabitsuki was unable to access, allowing her to find three collectibles exclusive to her known as Empty Boxes. Collecting all three of the Boxes is a requirement for accessing the game's final, true ending.
Rust curiously seems to either be a Fetus or is turning into one: If her sprite when severely damaged is closely examined, she appears to have horns identical to theirs. She also notably does not pinch herself to wake up, instead facing forward as the screen dissolves into red static. She also, curiously, is unable to sit down, either on the ground or on any chair. The sole exception is her computer chair in Sabitsuki's Room used to access flow.
Spoilers end here.
Similar to Yume Nikki and most of its fangames, not much is revealed about Sabitsuki's past explicitly in the game. Little to nothing is known for certain about her. When prompted to leave her room she shakes her head, in a similar fashion to Madotsuki, possibly implying she's a hikikomori or being quarantined. The dreams she experiences tend to be dark and violent, suggesting her personality or past experiences could be of the same nature.
Throughout the game, there are motifs involving illness, hospitalization and social exclusion, possibly suggesting that Sabitsuki is/was at some point in her life affected by a deadly disease of some sort, possibly one that may have separated her from her peers at school. It can therefore be theorized that she may be an excluded, introspective individual. Whether this is just a consequence of the illness/disability she has learned to deal with, or Sabitsuki simply longing to reach out to other people, is unknown -- though it should be noted that the vast majority of the humanoid figures she sees in her dreams have parts of them decaying or (badly) injured.
- According to the game's equipment screen, her messy hair stems from "bed hair," her tank top is her "usual outfit," her skirt is her "favorite," and her shoes are made out of leather.
- Sabitsuki appears to be left-handed: If one looks carefully, her computer mouse is located on the left side of her desk setup. She also uses her left hand to hold the Iron Pipe, Watering Can, and Handgun Effects.
- lol has confirmed a variety of information about Sabitsuki on their marshmallow-qa account:
- Sabitsuki is around 140-150cm tall, or between 4'6" and 4'9" roughly.
- Sabitsuki dislikes spicy food. She also frequently gets nosebleeds.
- Sabitsuki appears to have a mild interest in fashion and is interested in getting a navel piercing.
- Sabitsuki likes to listen to electronica and rock music.
- Sabitsuki's favorite colors are orange and red.
- Sabitsuki vaguely smells of blood. 
- Sabitsuki's favorite food is cake. Her favorite drinks are either coffee or tea.