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.
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.
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.
- 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.