Mayuri
Kurotsuchi

Mayuri Kurotsuchi is the captain of the 12th Division, head of the Shinigami Research and Development Institute, and the show's standing argument that Soul Society was never the good guys. Urahara released him from the Maggots' Nest, the prison for Shinigami too dangerous to walk free, because he needed a mind with no brakes. Mayuri has spent every arc since proving the assessment: he dissected Quincy corpses for research, turns his own body into a trap, and treats every battlefield as a data collection problem.

His fights are horror setpieces. Against Uryu he unveils Konjiki Ashisogi Jizo, a Bankai that exhales poison through a giant caterpillar with a baby's head. Against Szayelaporro Granz he wins so completely it becomes philosophy, killing the self-proclaimed perfect being with a drug that stretches one second of pain into a subjective century. His daughter-creation Nemu absorbs his abuse for the entire series, which makes the Blood War's turn land harder.

That arc is Mayuri's strangest showcase. He saves the Gotei 13 twice with preparation nobody asked for, zombie-curing drugs for Hitsugaya and the others, modified Arrancar as shock troops, and he outfights Pernida Parnkgjas, the Soul King's living left arm, after it mangles Kenpachi. The cost is Nemu, who throws herself into the fight to save him and is devoured. Mayuri, who insists attachment is a research defect, recovers her remaining brain matter to grow her again, and the anime leaves him sleeping in a preservation pod in his lab, having spent more of himself in Wahrwelt than he will ever admit.