Secret Identity Time

Following the play, Kayano lures Nagisa to the storage shed and, once Korosensei joins them, unveils tentacles growing from the back of her neck. She springs a pitfall trap into a pool of anti sensei pellets, her tentacles reading Korosensei's every move, and he is forced to blast his way free. Emerging changed in both look and manner, she names her target's crime plainly: she wants vengeance for the killing of her sister, Aguri Yukimura, and departs promising a later battle.

Elsewhere Shiro recalls being stunned to recognize this student, and explains that Kayano is Akari Yukimura in disguise, a gifted child actress who obtained a vial of the tentacle weapon and implanted it in herself. The stunned class pieces together how she masked her bloodlust by staying close to Nagisa, and Itona notes that maintaining such tentacles demands agonizing, relentless training.

Kayano's flashback traces her path to revenge: after an explosion at her sister's workplace, she found Aguri dead beside a tentacled figure and a note pointing to Kunugigaoka Junior High. She took the vials, transferred into the school, wrecked the Chairman's laptop to land in Class 3-E, dyed her hair green, and reshaped Nagisa's hair into his trademark pigtails. Shiro accuses her of jumping ahead of his own plan, and she coldly rebuffs him, reminding him that he abandoned Itona and that she works alone.

Pressed by his students, who threaten to quit unless he speaks, Korosensei agrees to reveal his past, but only to the whole class and only once Kayano is brought back. The group gathers at a park at seven, where he warns Kayano the tentacles are killing her. She refuses to relent, insisting the entire year was an act, and renews her attack. Within fifteen seconds her tentacles begin overtaking her mind, and her buried self silently pleads for help.

The adaptation differs from the manga in several details, softening the pitfall walls to bare soil, leaving Nagisa bruised and needing Isogai's help out of the shed, and shifting when the ground shakes. It also changes how Kayano destroys the Chairman's property on her first day and has the class watch her old shows on their phones rather than through Ritsu's monitor.