Abandoned by her parents, Shiro is an 11 year old genius NEET (Not in Education, Employment, or Training) hikikomori (shut-in) gamer who, along with her step-brother, Sora, form 『 』 (Blank). Sora's dad married Shiro's mother, both having remarried, thus making Shiro and Sora step-siblings.
Shiro is a pale-skinned girl with very long messy white hair with a pale blue, purple, pink, green, and yellow tint and large, passive golden/yellow-red eyes. She is rather slim and about average height (for an eleven-year-old).
She often wears a purple sailor uniform which may be a bit oversized, along with thigh high stockings. Her mouth usually resembles that of a cat's.
Shiro is described as a genius in logic and problems, but has difficulty understanding emotions or behavior, relying on Sora to help her defeat beings with emotions such as Tet in their chess match. She also rarely shows much emotion at all and, despite her genius intellect, speaks in short, terse sentences and in a third-person perspective.
Shiro is a master of Chess and has won every chess game that she has played. She can speak 18 languages and learns Imanity's language in under 15 minutes (while Sora needed an hour). She is also excellent with games and, along with her brother, is the best in more than 280 games.
She is shown to have a dry and somewhat twisted sense of humor, displaying signs of jealousy if Sora pays serious attention to other girls, showing she has a brother complex. She is also shown to be very sarcastic when it comes to Stephanie, although she calls her "just a friend". She is also not above teasing Sora about his mildly perverse tendencies.
Shiro is extremely close to her brother, to the point of requiring him to be by her side in order to function correctly. If Shiro is separated from Sora by so much as a closed door, she becomes almost catatonic until he returns to her. This can generate problems as when Shiro bathes; she does not want Sora to see her naked yet she can't be separated from him far enough to not be seen. This can be resolved with the presence of steam to cloud their view enough. However, Shiro also generally dislikes bathing as it stiffens her hair, although this was resolved by the Spiritwater Blend shampoo from Jibril.
She is shown to be a little bit rough and competitive when it comes to games and tends to show-off for any reason possible for the game.
Shiro also has a slightly sadistic side to her, which she usually keeps hidden. This is shown evident when she forces Plum to lick the sweat off her feet in order for him to regain energy. Although Plum does not think too badly of it, Sora still can't help but be shocked every time he sees his sister act that way.
Shiro has shown to be a genius with an almost supernatural intelligence, with incredible scientific calculation and simulation abilities. Like her brother, she possesses eidetic memory and is able to learn and recall a vast amount of information quickly and accurately. Sora has stated that she has beaten a chess program written to beat a grandmaster twenty times in a row, and she herself has stated that chess is no different from Tic-Tac-Toe. It has also been stated by Sora that she fluently speaks 18 languages, and learned to read Imanity in 15 minutes. She is also incredibly skilled in first-person shooters due to her ability to predict the movements and bullet trajectories of her opponents. These predictions are more focused on mathematical probability rather than psychological observation and deduction, unlike Sora.
While a genius, Shiro has trouble dealing with the human aspects of games, like reading human opponents and predicting their moves. Another of her weaknesses is the fact that she can't swim, and is afraid of snakes. A human player doing something unpredictable can unsettle and confuse her, and she relies on Sora to tell her when a human opponent is leading her into a trap, as shown in their chess game with Tet.
She is also very weak even compared to Human/Imanity girls at same ages with her. This is a result of her already-petite body and years of unhealthy lifestyles. Shiro make-up by keeping her physical works at minimum and left said work to others.
Her other weakness is her reliance on Sora. She must be within range of sight and/or sound to Sora or else she'll fall into an almost catatonic state, as shown multiple times in both anime and manga.
- Shiro means white in Japanese, which is possibly a reference to her albino appearance
- She wears short skirts that gives her a tendency to pull it down from time to time.
- She is afraid of snakes, and can't swim.
- Shiro and Sora's surname was said to be "Nai" by the author. This can be interpreted that their surname is literally Nai or that they have no surname, as Nai (ない) in Japanese means none (which is likely a reference to 『 』).
- Her name is pronounced similar to Zero, which becomes more clear when her name is spelled in English/Imanity (Siro) ("Si" is another way to romanize "Shi")