Isami Ōmiya is the older sister of the main character Shinobu Ōmiya and is introduced during her final year of high school before later moving on to college. She works as a fashion model, a profession that suits her graceful demeanor, and she possesses considerable knowledge about posing and photography techniques related to this work. Her appearance is distinguished by her long, dull-green hair, which is styled in a hime cut, and her matching green eyes. She has a tall and elegant stature that attracts attention.

In terms of personality, Isami comes across as a wiser and more mature version of her younger sister. She is fundamentally optimistic, kind, and caring, particularly towards her sister and her sister’s friends. She naturally adopts a big sister role, partly due to what is playfully implied to be a little sister fetish, and she often looks after the younger girls when their parents are away. However, this caring nature is balanced by a mischievous and teasing streak, and she often participates in playful banter with Shinobu and Alice Cartelet for her own amusement. Despite her elegance, she can also exhibit moments of laziness, such as when she avoids helping with baking by claiming her only dish is a burnt plate of eggs. She is not afraid to be blunt or stern when the situation calls for it, though her own moral code can sometimes be questionable. A recurring and quirky aspect of her character is her emotional dependency on being needed; she becomes deeply distressed or depressed when Shinobu does not rely on her.

Isami's primary role in the story is as a supportive older figure whose home serves as a central gathering place for the group. Her motivations are largely driven by her affection for her sister and Shinobu's friends, and she takes proactive steps to ensure their well-being and growth. Her most significant relationship is with her sister Shinobu, whom she adores and cares for deeply. Their dynamic involves a mix of genuine support, playful teasing, and occasional stern lectures, such as demanding to see Shinobu's grades. With Alice Cartelet, Isami is kind and often honest, but she has no issue teasing the girl or manipulating Alice's strong affection for Shinobu to get what she wants. She is greatly admired by Karen Kujō, who is fascinated by Isami's career as a model and frequently visits the Ōmiya household to interact with her. Isami shares a mature and easy understanding with Yōko Inokuma, having known her since childhood; she has entrusted the care of Shinobu to the responsible Yōko and even acts as a surrogate older sister for her, knowing Yōko wished for one. In contrast, she interacts the least with Aya Komichi, as they rarely see each other outside of group settings.

Over the course of the series, Isami develops from a high school student into a college student. Her emotional journey includes moments of vulnerability where she feels unnecessary when her sister becomes more independent, and she must rely on her younger friends to help stabilize her during these times. Beyond her modeling expertise, which is her most notable ability, she is also shown to be resourceful in managing the household and various situations, even if she occasionally prefers to avoid certain chores.