Shuuichi, also known by her nicknames Shuu and Nitorin, is one of the two main characters in the series. At the start, she is a young trans girl in the fifth grade of elementary school. Many characters describe Shuuichi as cute, and she can easily pass as a girl thanks to her feminine-looking face and physique. Shuuichi loves wearing clothes that enhance her cuteness. Her friends, Yoshino Takatsuki and Saori Chiba, encourage her to dress and behave in a more feminine manner. As Shuuichi progresses through puberty, she becomes increasingly worried about bodily changes such as body hair growth, a deepening voice, and acne.
Unlike most children her age, Shuuichi shows little interest in typical activities like sports. Instead, she prefers indoor pursuits. One of her favorite hobbies is baking sweet treats, although she is never shown preparing other types of food. After starting an exchange diary with Takatsuki, Shuuichi develops a passion for writing stories. This leads her to join the drama club as a writer in junior high school, following a co-written rendition of Romeo and Juliet performed by their classmates.
Shuuichi has a very honest personality, which helps her get along well with others. She usually exhibits an obedient and dutiful attitude. However, as she matures and experiences puberty, she sometimes has emotional outbursts that contrast with her typically calm demeanor. Shuuichi can be overly sensitive at times, often crying in front of others. Due to her personality, it is easier for her to form friendships with girls her age, resulting in her having very few male friends.
