Korosensei

In his monstrous form Korosensei looks like a yellow octopus roughly three meters tall, crowned by a swollen head locked in a wide grin that hardly ever shifts, with tiny beady eyes below it. Two of his rubbery, jelly-like limbs serve as arms, each ending in a pair of finger-like feelers, while the remaining tentacles carry him along the ground. A concealed pair of nostrils can give the impression of four eyes when noticed, and because his smile stays frozen, he signals his feelings through changes in color. His outfit is a black scholar's gown with a small square cap and yellow tassel, plus a broad tie marked by a yellow crescent moon that Aguri Yukimura gave him.

Back when he was human, he had light skin, a lean frame, dark eyes, and cropped black hair falling to the nape of his neck, together with a friendly smile he first wielded to deceive people. Formal wear was his choice on assassination jobs. Once Kotaro Yanagisawa held him captive, the prison uniform he wore was hospital scrubs, which he kept on right up to the moment he broke free and transformed.

He grew up somewhere so poisoned by betrayal that trusting anyone but himself felt impossible, and that upbringing pushed him toward killing as a means of survival; for years he regarded even his apprentice as merely a useful instrument. Knowing Aguri Yukimura finally showed him what compassion was and turned him genuinely gentle. As the class first meets him, he is loud and clownish, demanding trivial favors even while insisting he will destroy the planet within the year, and he flips from goofy to grave in an instant. Nagisa records that he can be quietly spiteful and that he lives for gossip.

What matters most to him, though, is the welfare of his pupils. He guards them ferociously and boils over whenever they are put at risk, even at each other's hands, tuning himself to each student so precisely that he will step in to fix their private troubles. That devotion earns their respect despite the doom he represents. Streaks of his old Reaper self still break through, as when he coldly threatened the students' families after one boy was nearly used as a living bomb. His cravings for sweets and his shameless perversion both trace back to the tentacle experiments, which stripped away his ability to mask what he truly feels.

Born without a name in a corrupt slum, he came to believe that only death, money, and raw skill could ever be relied upon, and he rose to become the storied killer the world called the Reaper once his tally of kills passed one thousand. He bested stronger foes with intellect, sharper foes with muscle, and the rest with charm, slipping past even the most fortified security. His downfall began when the orphaned child of one of his victims begged for training; he accepted the boy purely for the openings an apprentice provided and treated him as a mere tool, and in time that student turned on him and left him to be captured during a prison break.

Inside Kotaro Yanagisawa's hidden laboratory, where antimatter was cultivated within living cells, he grew close to Aguri Yukimura, the mistreated woman wed to his captor. She taught him tenderness and gave him the crescent-moon necktie for a made-up anniversary. When a lunar test showed that a creature carrying his cells would detonate upon dying, the researchers calculated that he, and therefore the Earth, would blow apart the following March. Loosing his newfound powers, he tore free in a rampage that cost Yanagisawa an eye, but a booby trap intended for him mortally struck Aguri. Her final request was that he take over teaching Class 3-E, so he shouldered blame for the moon, warned world leaders of the coming catastrophe, and accepted the post on the condition that his students remain unharmed.

His altered body renders him nearly indestructible: he races at Mach 20, mends severed limbs, throws afterimages, takes flight, recolors his skin, and even operates at the cellular level, though the heart tucked behind his tie stays lethal and seizing every tentacle at once leaves him helpless. Over three terms he lifts up Class E, overcomes Itona and chairman Gakuho Asano, weathers Kayano's vengeance, and lays bare his history. Cornered at last by the government's Spear of Heaven and betrayed once more by Yanagisawa and his former apprentice, he rescues Kayano and permits his class to hold him down. Following one final roll call, Nagisa strikes the fatal blow, and Korosensei scatters into motes of light.