Koro-Sensei’s Human Form & Backstory!

Koro-sensei has been the only one to truly see the students of Class E for who they really were and did everything he could to bring out all of their talent, support, and encourage them.

While everyone else was busy focusing on their negatives and being behind in academics, Koro-sensei really helped them find and hone their skills. But you know, he wasn’t always like this.

In fact, Koro-sensei started as a handsome human heartless assassin. As a human, Koro-sensei was quite an attractive man. With fair skin dark eyes and short raven hair, Koro-sensei was quite the looker!

He also made sure to always have a nice, kind, and friendly smile on his face. This smile was just a farce to hide the deadly assassin that he was.

He wore formal clothes while on the job, as a regular salaryman would. He also managed to look attractive in the patient scrub that he had to wear when he was captured.

Streaks of his assassin personality peep through when he gets angry and emotional. He then resorts to his default teaching technique, which is through fear rather than empathy.

How did he come to be the Koro-sensei we know and love?

Here’s the story of how Koro-sensei became Koro-sensei

Korosensei Human 1 Koro-Sensei's Human Form & Backstory!

Koro-Sensei Born

A child was born in a slum where he learned and trusted only one thing.

If you kill a person, they die. This child, had it grown up in a better environment, would have been kind and compassionate.

This child took the only path that he knew and believed was the path of an assassin.

The Reaper

This assassin was adept at dealing with any kind of enemy. Kill the strong with intellect. Kill the smart with strength and technique.

And those who were blessed with both, fell victim to his charm. This assassin had successfully killed so many people that he earned himself the name ‘The Reaper’.

The reaper continued to kill until one day he found a boy who requested to be his student.

The Betrayal

The Reaper taught his student any skills that he had, including all of his knowledge, skills, and techniques. His student performed well alongside him.

The Reaper, however, did not trust him and only saw him as a tool to use for his jobs. During one of their assignments, the Reaper was proved right. His student betrayed him and framed him while getting away himself.

The Experiments

An orphan with no official existence and his work as an assassin, nobody would care if he died. The Reaper was captured and made into a test subject by Yanagisawa, who was experimenting on antimatter.

He regularly injected antimatter into the Reaper’s body to test its effects on him and his body.

Slowly and steadily as the amount of antimatter in his body went on increasing, the Reaper’s body started reacting to it in various ways.

He obtained superhuman powers like super speed, poison immunity, absolute defense form, regeneration, and so on.

The Hope

The Reaper in the laboratory met Aguri who was engaged to Yanagisawa. Initially, he did not think of her as anything, not even a hostage since she didn’t have a loving relationship with her fiancé.

As time passed on, Aguri and the Reaper continued talking, she understood his kindness, and he understood that all of her flaws were a part of her charm.

Aguri mentioned that the Reaper would be someone a little perverted with a good heart, a little petty and stubborn if he didn’t need to go through all the hardships that he did.

She was a teacher, who often talked about her students and how she wanted to help them but couldn’t.

She also mentioned how the Reaper could actually be a great teacher. She was the first realization, the Reaper had, of what kindness and compassion meant. But that wouldn’t be for long.

The Death of Koro-Sensei

Yanagisawa launched an experimental test to the moon for the test of antimatter in cells. A lab rat was injected with antimatter and sent to the moon.

This caused 70% of the moon to be destroyed. Yanagisawa realized that essentially, what he had made was a living time bomb.

This meant that the Reaper was a time bomb that would go off in another year. He immediately ordered the Reaper to be killed. Aguri who had secretly heard all of this rushed and told the Reaper everything.

The Reaper calmly told her that he anticipated death soon. It caused him to go on a violent rampage killing everyone in sight.

Yanagisawa tried his best to stop him but because of the abilities that he had gained, the Reaper was now unstoppable.

The Reaper planned to go down to the earth and destroy it. Aguri clasped onto him from behind, stopping him.

This, however, triggered a trap in the ruined building, and a tentacle pierced straight through her abdomen.

In her dying breath, he pleaded with the Reaper to help the kids of Class E since he would be a great teacher.


The Reaper was greatly saddened by her death but didn’t lose hope. He offered to be the teacher for Class 3-E. He took the crescent moon from Aguri’s top and attached it to his own tie.

He donned a graduation cap with a yellow tassel and a coat. Until now, the Reaper had received his birthday from Aguri but not a name.

Sensei entered the class, introduced himself as someone they had to kill but couldn’t, and asked the students to name him. Kaede then shortened ‘Korosenai Sensei’ or ‘Unkillable teacher’ to ‘Koro-sensei’.

This is how Koro-sensei became the Koro-sensei that we love.

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About the Author

Garima Singh

Moshi Moshi everyone! I'm Garima, an anime enthusiast with a passion for sharing my love of anime with others. I've watched over 1000 anime, and I'm always on the lookout for new and exciting shows to watch.

I love everything about anime, from the stories and characters to the animation and music. I'm also a big fan of anime culture, and I love to learn about Japanese culture and history through anime.

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