Shuuichi, otherwise known by her nicknames Shuu and Nitorin, one of the two protagonists, is a young trans girl, who at the beginning of the series, is in the fifth grade of elementary school. Shuuichi is described as cute by many of the other characters, and is able to pass as a girl due to her feminine-looking face and physical build. Shuuichi enjoys wearing clothes which make her look cute, her friends, Yoshino Takatsuki and Saori Chiba, encourage her to dress and act femininely. As Shuuichi grows up, she becomes increasingly concerned about the changes her body is undergoing due to puberty such as the growing of body hair, her voice becoming deeper, and the onset of pimples.

Shuuichi does not show any outward interest in other activities akin to a child her age, such as playing any sports, and is usually seen doing indoor activities. Shuuichi enjoys and is skillful in baking sweet foods, though she is never shown cooking other more conventional foods. After starting an exchange diary with Takatsuki, Shuuichi becomes interested in writing stories, and even joins the drama club as a writer with Saori in junior high school after they co-write a rendition of Romeo and Juliet performed by members of their class.

Shuuichi has a very honest personality, which generally makes it easy for her to get along well with others. She tends to have an obedient, dutiful attitude, though as she grows up and enters puberty, she sometimes experiences outbursts of emotion which contrast her usual quiet nature. Shuuichi can be overly sensitive at times, and is often shown crying in front of others. Due to her personality, it is easier for her to associate with girls her age, and as such she has very few male friends.