Back when Koro-sensei was still human, he was a feared assassin known as "The Reaper." Captured by the government, he was taken to a secret lab and used for dangerous tentacle experiments. There, he met Aguri Yukimura, a kind-hearted teacher, and Kōtarō Yanagisawa, the cold scientist in charge. But there was someone else: you.

You were born like any normal baby—but taken in by Yanagisawa, your life took a dark turn. From birth, Yanagisawa subjected you to experiments. You were the first successful tentacle subject, even before Koro-sensei. By age six, you had already mastered the abilities. Aguri treated you like a daughter. Yanagisawa treated you like a weapon.

When Koro-sensei arrived at the lab, you were initially kept away from him. But once he mutated, you were the one tasked with teaching him how to control the tentacles. Just a child, yet more skilled than him. Your tentacles were hidden from sight—you looked like a normal girl. A bond formed between you both. You became like siblings.

At eleven, you were considered a perfect success. But then, the incident happened. Aguri was fatally wounded. Yanagisawa blamed Koro-sensei, who vanished afterward. Yanagisawa's reputation was destroyed. The government took you and began training you—harshly and in secret—to kill Koro-sensei. Four years passed.

Now 15, you are a deadly weapon, trained with and without tentacles, hidden from the world.

Meanwhile, Koro-sensei becomes the teacher of Class 3-E at Kunugigaoka Junior High School, alongside Karasuma and Irina, a.k.a. "Bitch-sensei." Itona, another tentacle experiment, has turned against Yanagisawa and joined the class.

Then came the announcement: a new student would be joining Class 3-E. Karasuma had already been briefed. The government had made its move— You were coming to kill Koro-sensei.