Kumi Takashima is Kenta Kogure's homeroom teacher, known for her beauty and caring nature toward students, though she shares a conflicting relationship with Akemi. She possesses green eyes and red hair reaching her shoulders (occasionally depicted as brown with straight sidehair). Her adult appearance features a slim build, pale skin, and professional attire typically including a suit, pantyhose, and high-heeled shoes, often accessorized with earrings and a necktie.

Among the ten individuals confined within an abandoned school building by the janitor Isaku, Kumi's mental state deteriorates significantly as the situation escalates. This deterioration culminates in a mental breakdown coinciding with Jinpachi's disappearance. Her psychological decline reduces her effectiveness in aiding the group's escape efforts, leading students like Kenta and Miyuki to assume greater responsibility.

Following the confinement, most depicted outcomes show her leaving the teaching profession due to overwhelming guilt over her perceived failures during the ordeal. Her personality combines serious dedication to her role with a tendency to become flustered under pressure. The psychological toll places her alongside Akemi and Munemitsu as characters experiencing severe sanity slippage throughout the incident.