Rein has purple eyes (blue in the anime) and medium-length black hair in a bob cut. She usually wears a light-colored shirt, a black one-piece outfit with a skirt, and dark stockings. Her casual wear consists of a shirt and short shorts, though she is sometimes shown without the latter.
When fighting or participating in Darwin's Game, she wears a large, light-colored coat that covers her body from the shoulders down. After the timeskip, Rein wears a hooded jacket similar to her coat, along with black tights and shoes.
Despite her age, Rein is very mature, is a voice of reason, and is highly insightful. She is usually the primary source of information for her clan and is reliable when coming up with strategies when the time comes as she is analytical. She is able to keep calm in tough situations, such as when she insulted Wang despite him holding her. She prefers to stay indoors and was shown with a displeased expression when Nayuta mentioned that the Out of Range Village had no internet.
She dislikes being referred to as a child, sometimes reminding the person who said it that she's a first-year in middle school. After the timeskip, she modifies said phrase to mention that she's an adult. Rein's motto when it comes to Darwin's Game is to run instead of trying to fight or capture an enemy, which is how she's survived until now. Nevertheless, she resolves to fight as a member of the Sunset Ravens in order to protect her new home. Rein is also concerned that her jokes are hard to understand.
Unlike Shuka, who is true to her emotions, Rein is usually calm and deals with things logically. She mentions that it feels like two Reins always exist inside of her; one appeals to her emotions while the other calmly observes, likening it to depersonalization. Because of this, she is shown to be calm in emotional situations, recalling that she didn't cry when her parents got divorced. Although she admits that she feels jealous that Sui could express her emotions so straightforwardly, she also acknowledges that her calmness has helped her a lot in surviving the Darwin's Game.
Two years before the series, Rein dreamt of sinking in an ocean after receiving news from her parents about her father's new job and their divorce. She stayed behind with her mother in Japan as she is shown attending a Japanese middle school.
Sometime before the series, she was invited to the Darwin's Game by her classmate Tachikawa Mio and gained Laplace, using it to save her friend from being killed by a shard of glass.
Skilled Sniper: Rein is skilled at sniping and is her preferred combat style when she has to fight. In a CIA report, she is stated to have succeeded in hitting a target from more than 2000 meters away while in combat.
Athletic Ability: After the timeskip, Rein is much more athletic, flipping from a bus window to the roof, and was shown jumping off of a greed while fighting them.
Computer Skills: Rein is shown to be good at using a computer, being able to program from a young age. She also developed a program to sync information from the Darwin's Game app to her workstation and display it.
Equipment: Rein is shown to use several different sniper rifles which she keeps in a case she carries around when she has to fight or participates in the Darwin's Game. She also owns a Skorpion vz. 61 which she was shown using during her clan's battle against Liu Xuelan.
- According to the anime booklet, Rein's birthday is May 1 and her blood type is A.
- Rein likes Umaibou, especially the nattou flavor. She keeps boxes of it in the data analysis room which she claims is her source of energy.