Koro-sensei

Koro-sensei is an incredibly powerful, octopus-like being who stuns the world when he turns the moon into a permanent crescent, then declares that he will wipe out the planet within a year. Instead of striking right away, he takes a strange detour: he becomes homeroom teacher for Class 3-E, a group of students the rest of their school has given up on, while the government quietly asks those same students to kill him before his deadline arrives, offering a massive reward for success.

Despite that monstrous threat, Koro-sensei turns into a genuinely devoted teacher, paying close attention to each student's personal troubles and helping them grow, both in regular lessons and in the assassination training built around him. His odd classroom nickname is actually wordplay in Japanese, built from a phrase meaning he cannot be defeated, so his students essentially call him "the teacher who can't be killed." Flashbacks later reveal that he once lived as a feared human assassin before a betrayal by his own apprentice and a set of experiments left him in his current, inhuman form.

He made a quiet promise long ago to a researcher named Aguri Yukimura, who treated him with kindness before she died, and raising Class 3-E grows directly out of trying to keep that promise. By the finale, Nagisa Shiota, the student he has bonded with most, is the one who finally ends his life, carrying out the task Koro-sensei wanted completed all along.