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Ansatsu Kyoshitsu vol. 16, by Yusei Matsui, Grade B+
(The real Shinagami.)
There was a kid in the slums somewhere. Life was tough, with best friends and lovers betraying everyone, and dirty cops killing clean cops over money or drugs. The kid quickly learned that there was only one thing you could rely on - if you killed someone, they would die (and not come back). He became a killer, and over time mastered the arts of assassination. He looked gentle and harmless, which was just one more trick for getting close to his target. He fought everywhere, in offices, on the street, and in wars. When his body count topped 1,000, he was given the nickname, "God of Death" (Shinagami). He decided to take on a student in order to get the jobs done more quickly, but the kid set him up in a trap, saying that since he'd learned all there was to know about assassinations, he was going to become the New Shinagami. Then he took off his mask to reveal a skull face, and left. The old Shinagami was locked up in a research lab as a guinea pig.
(Aguri likes to live dangerously, in a "black demon tube top".)
The lab was headed by Kotaro Yanagisawa, an incredibly arrogant researcher working on anti-matter bio-weapon development. The idea was to inject specially treated anti-matter into the subject, and have it race through a ring of cells kind of like an accelerator, to morph the cells of the host's body, giving him or her greater strength and abilities, and possibly turning them into an unlimited electrical generator. One of his assistants was the naive woman, Aguri Yukimura. Aguri was brought in to teach the 3-E class at some junior high school, and she really tried her best, but the school chairman had designated 3-E as the bottom of the school, and the students were resigned to never get any better. Additionally, Yanagisawa would scream at, and beat, anyone that he thought had screwed up, which included Aguri. Shinagami, who Yanagisawa dubbed "Morumotto" (guinea pig), watched all of this from his clear acrylic-walled cell, and waited for his opportunity to escape.
(Koro goes full-blown tentacle.)
Aguri liked wearing childish t-shirts under her work clothes, and both her students and Shinagami would comment that she had a poor fashion sense. Additionally, she was working 16-18 hour days, and would spend her down time at the lab writing up homework and test questions for her students. Shinagami had studied physics for one assassination job, and he'd correct her mistakes by breathing on the wall and writing the solutions in the mist. Eventually, he found himself being attracted to her, and after being in the lab for one year, Aguri brought him a "birthday" present - a fat, black necktie. She wanted to be able to hold him, and Shinagami revealed a secret - the lab research had made greater progress than anyone else had realized. He used his tentacles to drill tiny holes in the cell wall and extended them to make a form of hand to hug her.
One day, one of the other research subjects had a major result failure. Yanagisawa had injected a mouse with the anti-matter juice, and it had been left in a self-contained lab module on the moon. The anti-matter cell accelerator process broke down, and the subsequent explosion destroyed 70% of the moon. Yanagisawa ran the numbers, and figured that the same thing will happen to Morumotto in March of next year, which will completely eliminate the Earth. He ordered his security guards to kill Shinagami, but Aguri overheard the conversation and warned the assassin first. He activated the full powers of his tentacles and easily escaped his cell and killed the guards with their own bullets despite the fact that the pellets were coated with anti-tentacle material (by simply manipulating the air around the pellets). The power started to go to his head, and he prepared to go outside. Aguri realized that this would be bad in his current state, and she ran up to stop him. In doing so, she put herself in the way of a booby trap (Yanagisawa had applied the tentacle seed to chameleons, and used their tails as electrically-triggered tentacle launchers). The booby traps were designed to sense Shinagami's presence, and one of them launched and speared Aguri through her chest. As she died, she got Shinagami to promise her to escape, cool off, and take over her classes for her at the junior high school. That's when her younger sister, Akari (Kayano) arrived and saw the monster standing over her older sister. Shinagami left a note on the body telling Yanagisawa where to find him, took the present Aguri still had in her lab coat, and he left.
(Aguri tries to stop Shinagami from leaving the lab, while in a towering rage.)
Shinagami flew off to make a secret cave base for himself, and used his tentacles to harvest materials from the field around him to sew his own teacher's uniform and cap, and put a Moon logo over the puncture hole in the necktie. He then reformed some of the tentacles around his face and emerged as Koro-sensei.
After Koro finishes his story, the kids flashback to the highs and lows of the past year, and they're starting to realize the impact of their lessons. If they don't kill Koro by March, he will explode like the mouse did, and wipe out the planet. As far as Koro is concerned, he doesn't care if he goes out alone, or takes everyone else with him. Nagisa rallies most of the other students together to try to find a way to save Koro. Nakamura, the manipulative girl, and Ryouma plus his gang take the opposite position that Koro must still die.
Summary: And now we know the rest of the story. Yanagisawa is Shiro, and he's still doing tentacle research. His main gripe with Koro is simply that he doesn't like ANYbody. Koro probably did fall in love with Aguri, but it's impossible to confirm now. And now, the class is about to fracture into a civil war. Highly recommended.
Inside cover quote:
This is a new pattern from Koro-sensei.
After selecting this royal tartan check, he suddenly called it "Koro-check."
From a muffler design to an idol skirt, we can see Koro-sensei everywhere.
Back inside cover quote:
There is a tentacle cell!!
- Koutarou Yanigasawa