Kasumi Tachibana is the elder daughter of Ichiro Tachibana, the head of the Takeshi organization's Kanto branch, which operates out of the Tachibana family sweet shop. She is a young woman of 23 years old who serves as a primary field support coordinator for the Oni known as Hibiki. In this capacity, she is responsible for logistical tasks such as transporting Hibiki to mission locations using the vehicle Shiranui, providing supplies, managing and maintaining his supply of Disc Animals, and assisting in the search for rampaging Makamou.

In terms of personality, Kasumi is depicted as a strong-willed and intelligent individual who possesses a sharper and more aggressive disposition compared to her younger sister, Hinaka. Despite her assertive nature, she is also noted to be feminine and fashion-conscious, managing the front of the Tachibana sweet shop when not out on assignments. She cares deeply for her younger sister, offering her advice and support, particularly regarding Hinaka's relationship with the Oni Todoroki.

Kasumi shares a significant and long-standing relationship with the Oni Ibuki. The two have been close friends since childhood, and Ibuki harbors romantic feelings for her. Kasumi is aware of this, but displays a tsundere attitude towards him, often reacting sharply to his indecisiveness while clearly maintaining a special bond with him. Early on in their working relationship, she tends to ignore his advances, but her underlying care for him becomes more apparent as the story progresses.

Her role in the story focuses on her unwavering support of Hibiki, ensuring he can focus on combat by handling the logistical and technical aspects of their missions. She is also the one who voices concerns about the potential consequences of civilian Asumu Adachi being exposed to the Oni’s world, showing her pragmatic and protective side. Throughout the series, Kasumi demonstrates her competence and dedication to the Takeshi organization, from dealing with dangerous Makamou to helping research old records for vital information, solidifying her role as a reliable and capable coordinator in the fight against the forces of darkness.