Itoko Osakabe teaches physics at Yagami High School while housing her cousin Kenji Harima, who left his family. Though their relationship remains distant, she quietly supports his growth, recognizing Tenma Tsukamoto’s stabilizing effect on him. Her stern classroom presence masks a playful tendency to tease Harima about his crush on Tenma, devising lighthearted traps that exploit his flustered reactions.

Linguistic irony defines her name: "Osakabe" merges "punishment" and "department," while "Itoko" (meaning "string child") homonymically echoes "cousin," leading to confusion when Harima mentions her.

Standing 168 cm with dark violet hair and grey eyes, her striking appearance draws admiration from male students in class 2-C, particularly for her figure—a fact that backfires when she reluctantly models nude for colleague Yoko Sasakura’s art exhibition, triggering campus-wide chaos. Her mysterious past, guarded from exposure and leveraged by Yoko, adds layers to her pragmatic persona.

Financially sustaining Harima, she enforces rent payments he habitually dodges, resulting in two temporary ejections from her home. Yet her subtle guidance steers his manga ambitions and pursuit of Tenma. Beyond Harima, she navigates friendships with colleagues like Yoko, joining spontaneous trips and school events that ripple through student dynamics. Though prioritizing Harima’s development, her actions during festivals or athletic meets subtly shape broader campus interactions, balancing duty with calculated mischief.