Shrieker was a serial killer in life who died shortly after murdering a woman named Shibata when her infant son Youichi tripped him out of a window. Stealing Youichi's soul and putting it inside of a parakeet, the Hollowfied Shrieker promises that if Youichi can escape him for three months at a time, he'll bring the child's mother back to life (something he can't do). Able to track Youichi's soul, Shrieker kills and devours anybody who tries to help Youichi, feeding off the psychological torment it causes, as much as the actual souls.
Mayuri Kurotsuchi is the Gotei 13's resident mad scientist who frequently conducts experiments on living subjects. He uses his unwilling subordinates as human bombs, and at one point blows up a survivor because bombs "aren't supposed to come back." He is also responsible for conducting grotesque human experiments on the Quincies, and keeps a picture of one of his mutilated victims on his person as a trophy. After the Soul Society Arc Mayuri no longer acts as an antagonist to the heroes, but he's still the most vile and depraved out of all the Gotei 13, and it's clear that he only works for the good guys for the chance to work on his sadistic experiments without repercussions. In fact, Mayuri was willing to kill all of his fellow Gotei 13 members by destroying the pocket dimension they were in just to take out Aizen. He was only stopped from doing this by his subordinates' interference. Mayuri also displays a callous disregard for the life of his "daughter" Nemu. He savagely beat her after she had been severely injured and, when an enemy had reduced her to a withered husk, Mayuri's only response was a spike of scientific fascination.
Sosuke Aizen is a sociopathic‹bermensch with a god complex who considers himself above the notions of morality and decency adopted by "lesser beings." Uncaring for anyone but himself, Aizen has a habit of using and betraying anyone close to him. Examples of this include testing out an experiment by turning his former captain, Shinji Hirako, into a monstrous abomination along with seven other shinigami officers, trying to kill his subordinate Harribel once she had outlived her usefulness, and impaling his devoutly loyal lieutenant, Momo Hinamori, while hugging her, leaving her traumatized for months. Later, Aizen would outdo himself by creating illusions that tricked Momo's best friend, Hitsugaya, into nearly killing her. Aizen also used Wonderweiss Margera as a tool, erasing all of his intelligence, speech, memory, reasoning and emotions just to turn him into a Power Nullifier against one man's weapon. Once Wonderweiss had successfully absorbed the flames of this person's weapon, Aizen had arranged it so that Wonderweiss self-destructed, nearly taking out a city with him in the process. Finally, when Aizen had achieved physical godhood, he tested out his abilities by calmly walking through a human city, knowing that his powers caused anyone who came into close proximity with him to blow up. In the end Aizen ended up killing over a dozen of civilians, and planned on doing the same to Ichigo's human friends just so he could mock Ichigo by hanging up their corpses on display.