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Tear Jerker: Ansatsu Kyoushitsu
The second half of chapter one reveals Koro-sense in his previous form with a woman, in a wasteland that looks like it was torn apart by some kind of disaster. She's the one who encourages him to be a teacher and seems to care for him and see him as beautiful despite not being human, but she's clearly dying.
Chapter 84. After losing to Koro-sensei yet again, Itona starts screaming in pain that his brain feels like it is tearing open. Shiro reveals that whenever Itona loses, it causes him great pain and he has to be brought back to a special facility for a few months to get it under control. Yet after stating this, Shiro tells him that he is abandoning him to move on to a more successful assasination strategy. The chapter ends with Itona lashing out before running away from Koro-sensei and the others, still screaming in unbearable pain.