Isami Ōmiya serves as the older sister to Shinobu Ōmiya, first appearing during their second high school year and later progressing to college. Her striking appearance includes long, dull-green hair styled in a hime-cut, matching green eyes, and a notably tall, graceful stature that draws attention. Initially seen in her high school uniform, she shifts to casual attire in later seasons.

Pursuing a career as a fashion model, she excels in posing techniques and photography insights. Her personality blends a protective, caring nature toward Shinobu and friends with a playful mischievous streak. She frequently teases Shinobu and Alice, leveraging their interactions for amusement. While generally kind and optimistic, she exhibits blunt or stern moments when necessary, such as demanding Shinobu’s academic reports or correcting inappropriate conduct. A recurring trait is her "little sister fetish," causing deep distress and depression when Shinobu fails to rely on her. Occasional laziness surfaces, like avoiding baking chores and declaring burnt eggs her signature dish.

Relationships define her familial and mentorship roles. With Shinobu, she balances affection with teasing, fretting over her well-being while playfully manipulating scenarios—admiring her independence yet crumbling when unneeded. She treats Alice with kindness and honesty, exploiting Alice’s attachment to Shinobu for fun. Karen Kujō idolizes her modeling career, leading to frequent interactions during Karen’s visits to the Ōmiya home. Yōko Inokuma, a childhood friend, shares a mature rapport; Isami acts as an older sister figure to compensate for Yōko’s lack of biological sisters. Aya Komichi admires her but has minimal direct contact.

Throughout the series, Isami transitions from high school senior to college student, enduring emotional slumps when feeling unnecessary to Shinobu. Friends rally to support her during these lows, stabilizing her emotionally—highlighting her role as both anchor and dependent within the group. Her Japanese name, meaning "brave" and typically masculine, pairs with the surname Ōmiya, translating to "big shrine/palace." Additional notes include her Capricorn zodiac sign, blood type O, and height advantage over peers.