Seiha has long, pale blue hair with choppy bangs that frame her face. Her expression is often flat, giving her a cool and collected aura. She has thick eyebrows and wears large-framed blue glasses. In her school uniform, she usually wears a black sweater over her top. She is the shortest of the Shiunji siblings and sometimes wears her mother’s ring, which she keeps safe.

Arguably the smartest of the Shiunji siblings, Seiha is the top-ranking student in her school and the president of the Science Club. She is highly intelligent and approaches almost every problem with strict logic, which can make her seem cold and distant. At times, her extreme rationality makes those around her uncomfortable, though she rarely seems to notice.

The plot surrounding Seiha Shiunji in The Shiunji Family Children focuses on her role as the highly intelligent and logical second-oldest daughter. As the valedictorian and Science Club president, she often approaches family and school matters with cold, rational reasoning, which sometimes clashes with her siblings’ emotions. Throughout the story, Seiha navigates the revelation that her siblings are not related by blood, balancing her analytical mindset with the emotional complexities of family dynamics and the growing tensions and misunderstandings among the Shiunji siblings.