Arata Shiunji is the protagonist of the story and the eldest son of the Shiunji family. He is a sixteen-year-old second-year high school student who shares a birthday with his sibling Ouka on October 10, and the two initially believe themselves to be twins. In terms of appearance, Arata is the tallest among his siblings and possesses a strong, athletic build that he has maintained since he was young, encouraged by his father. He has short, slightly spiked dark brown hair with the bottom half of the back of his head shaved. His facial features are defined by narrow eyes and thick eyebrows that are often slanted downward, giving him a sharp or potentially aggressive look that contrasts with his true personality.

Arata is characterized by a deeply protective and caring nature toward his family, which is partly driven by the burden he feels as the eldest male and the presumed heir to the Shiunji fortune. Despite his somewhat intimidating appearance, he is actually quite earnest and becomes flustered very easily. He is known for carefully thinking through his words and actions, though his intentions are frequently misunderstood by those around him. Arata firmly upholds the boundaries of his family, and each time one of his sisters approaches him in a romantic or sexual manner, he is quick to reject them, consistently reiterating that they are siblings regardless of blood relation. When he needs guidance, he often turns to his younger brother, Shion, for advice.

At the start of the narrative, Arata is navigating a seemingly ordinary life, resigned to the fact that he has never had a girlfriend despite being surrounded by his sisters. This situation is upended when his father reveals a shocking secret: the seven children are not all blood-related. This revelation forces Arata to confront a new and complicated dynamic as some of his siblings begin to see him in a different light. While he is the same age as Ouka, who was thought to be his twin, the news particularly strains that relationship, as the foundation of their identity as extensions of each other is suddenly thrown into question.

Throughout the story, Arata serves as a stable and serious anchor for his siblings as they each struggle with the fallout from their father's announcement. He is clever and takes his responsibilities seriously, often stepping in to help solve his sisters' personal problems. His motivation is not driven by romantic ambition; rather, he acts out of a genuine sense of duty as a good older brother who wants his family to be happy. However, this very kindness and reliability are what inadvertently deepen the romantic feelings his sisters develop for him. Arata has also been dealing with a one-sided crush on a girl named Rara Yokoyama, who already has a boyfriend. Too shy to make a move or even send her a friend request, he typically hides when he sees her, adding another layer of adolescent awkwardness to his life.

Academically, Arata is quite intelligent, earning good grades, and he is also proficient in sports, balancing his school life with his duties at home. His development largely centers on managing the chaotic new reality of his family while trying to maintain a sense of normalcy and sibling propriety, despite the shifting intentions of those around him. A notable piece of trivia is that, despite his status as the eldest son and heir, he has the smallest bedroom in the entire house because he lost a game of rock-paper-scissors.