Kasumi Kadomori is one of the twin sisters from the Kadomori family, guarding the Black Gate across generations. This gate acts as a passage to an alternate world possessing immense power, the Kadomoris' ancestral realm. As the firstborn twin, she shoulders the greatest responsibility among the eight "Gate Guardians," a duty ingrained since childhood. Her role demands preventing misuse of the gate's power and ensuring its security.

Her personality reflects this burden through diligence and approachability. At school, she participates in committee work, recognized for her dedication and friendly nature. Kasumi prefers thoughtful, principled action over strict enforcement, relying more on earned respect than assertiveness. Despite her kindness, she maintains a degree of reserve, extending even to her largely platonic relationship with her steady boyfriend, Narifumi Hosokawa.

A central conflict stems from her twin sister Shizuku's resentment. Shizuku feels overshadowed by Kasumi's inherited leadership role and the preferential treatment granted to the elder sister, growing bitter over their proximity to ultimate power without permission to wield it. This fuels Shizuku's ambition to seize the gate's power for herself, an act deemed sacrilege among the guardians. Shizuku exploits opportunities like the school culture festival to enact her plan, manipulating others by identifying vulnerabilities and eliminating obstacles. Kasumi's leadership faces its ultimate test as she confronts her sister's betrayal and the ensuing crisis threatening the gate's stability.

Physically, Kasumi has light brown hair, green eyes, stands 1.58 meters tall, weighs 47 kilograms, with measurements of 84-57-85 centimeters, and has blood type A.