Shion has short white hair and is almost always seen wearing a white beanie decorated with two blue stripes along the brim. Attached to the beanie is a large black button featuring the Crux constellation. He has large, slightly rounded blue eyes framed by long lashes, giving him a gentle and soft expression. When wearing his school uniform, he pairs it with a white hoodie under his blazer, adding to his relaxed and laid-back style.

Seemingly the most laid back of the siblings, Shion is frequently shown playing video games on a portable device or avoiding anything that looks like trouble. Although he gives advice to his older brother Arata when asked, he rarely takes the initiative on his own and prefers to stay relaxed and unbothered. In chapter one, he casually admits that he only had sex out of curiosity, wanting to know what it felt like. A female classmate, presumably his girlfriend Yuu, can be seen clinging to his arm in the background.

Among all the Shiunji siblings, Shion shows the strongest desire to learn about his birth parents. After discovering that he was born in Karuizawa, the same place where the Shiunji family owns a vacation home, he searched the town for information about them. Despite his efforts, he was unable to find any useful leads.

Shion Shiunji grew up as one of the youngest members of the Shiunji family, living a comfortable life alongside his siblings in their wealthy household. Although he was always seen as the most easygoing among them, he quietly developed a sense of independence and curiosity that set him apart. While his siblings focused on academics, social responsibilities, or personal goals, Shion often preferred to stay in his own world, spending most of his time playing games or relaxing.

His life changed after their father revealed that none of the Shiunji siblings were related by blood. Unlike some of the others who struggled with the emotional implications, Shion directed his energy toward uncovering the truth of his own origins. He learned that he was born in Karuizawa, which motivated him to search the town for clues about his birth parents. Despite his efforts, he found no clear information, but the experience strengthened his understanding of who he is beyond the family name.

Even with this unresolved chapter in his life, Shion maintains his calm and unbothered demeanor. His relationship with his girlfriend Yuu and his interactions with Arata and the rest of the siblings highlight his loyalty and perceptiveness, showing that beneath his laid back nature, he carries a quiet desire to understand where he truly came from.