In the Demon Slayer universe, the Twelve Demon Moons (Twelve Kizuki) are the twelve most powerful demons in existence who answer directly to the Demon King, Muzan Kibutsuji. Not only do they answer to Muzan, but they're also commanded by him too, and carry out his will as they see fit.
All of the twelve demons have one thing in common: they've received a large amount of Muzan's blood to amplify their abilities, and there are two major ranks, upper and lower. The Upper Ranks are far superior in power to the Lower Ranks, with the weakest being Lower Rank Six, but they're all still incredibly powerful opponents, typically only killable by the elite Hashira.
10 Misa Amane, Death Note, Lower Rank Five
Misa is lamented as having little drive or personality of her own in Death Note, a trait that's caused a large amount of discourse over just how good her character actually is, but something Misa lends herself well to is being entirely controlled by somebody else.
Her driving force is her devotion to Light, but if that devotion were to be to somebody like Muzan instead, she could become formidable in her own right. Misa was wasted because it was in Light's character to take advantage of her, the weaker she was the easier his life became, but Muzan would want her to be powerful, and she would do anything to please him.
9 Himiko Toga, My Hero Academia, Lower Rank Four
Toga joined the League of Villains after murdering a young boy at her school, mistaking her thirst for blood for a schoolyard crush. She showed little remorse for her actions and seems to find the most ecstasy in harming others, specifically making them bleed.
Not only is Toga a powerful opponent, and downright creepy, in her own right, she is largely devoted to her villain comrades and seeks to further their cause. In My Hero Academia, she's shown multiple times that she's capable of working as well on her own as she does with others, meaning she isn't too much of a liability despite her age and attitude.
8 Junko Enoshima, Danganronpa: The Animation, Lower Rank Three
Junko Enoshima wanted one thing and one thing only in Danganronpa, despair. Every single one of her actions directly contributed to causing an immense amount of pain and suffering in both her classmates and non-classmates alike, and her lasting legacy was a ruined world.
Being an agent of chaos and despair for someone else then wouldn't be too far away from Junko's typical day. Her particular penchant for harming others fits in well with Muzan's expectations of the Twelve Demon Moons, as he typically uses them to dispatch demon slayers who are causing him issues.
7 Nutcracker, Tokyo Ghoul, Lower Rank Two
Tasked with capturing humans to please an underground society of gourmet ghouls who were particular about their food, Nutcracker did more than just kill her victims, she tortured them. Obsessed with wealth and looks, those were her motivating factors beyond the torture and were certainly what she was thinking of when she killed her victims.
Not only was Nutcracker powerful in her own right in Tokyo Ghoul, but she also displayed a keen ability to work well on her own. With someone like Muzan being able to provide her with the beauty and riches she so desired, Nutcracker would've had complete loyalty to him and would've been unswayed by even Tanjiro's most impassioned speeches.
6 Erwin Smith, Attack On Titan, Lower Rank One
Erwin was not sadistic, cruel, or remorseless in Attack On Titan. He was an almost perfect leader who inspired legions of soldiers to follow his every command, he led the scouts with a thumping heart and plenty of passion, and his lasting legacy is still felt even by elite combatants like Levi and Hange.
But he could have a one-track mind, and if he were a demon who believed he was fighting for his kind, he wouldn't hesitate to perform some morally gray actions. Erwin was one of the only people who didn't hesitate after discovering the truth about the Titans, just as he wouldn't hesitate to kill demon slayers if he perceived them as a threat.
5 Frill, Wonder Egg Priority, Upper Rank Six
The artificial human who was shunned by the two men she believed were her family, Frill's story in Wonder Egg Priority is equal parts tragic and terrifying. After her physical body was destroyed, she continued to live on, now taunting other young girls into committing suicide.
Frill is hyper-intelligent and all she truly wanted was her family back. Though her actions were inexcusable, she hadn't been taught any better and she didn't understand how to control her jealousy. Muzan and the Twelve Demon Moons could not only provide her with a false sense of family, but Muzan could also control her insecurities to turn her into a very deadly demon.
4 Hisoka Morow, Hunter X Hunter, Upper Rank Five
The killer clown who tormented Gon throughout the first arc of Hunter X Hunter, Hisoka cares only about showing how powerful he is, and he shows this through defeating any strong opponents he can find. His need to fight strong opponents is so powerful that he says it's the only reason he became a Hunter in the first place.
He would fit perfectly then amongst the ranks of the Twelve Demon Moons, whose sole job is to take down powerful targets that pose a threat to Muzan. Hisoka wouldn't necessarily have the loyalty or drive to protect his superior as others would, but he'd be driven more than enough at the prospect of facing the almighty Hashira.
3 Noi, Dorehedoro, Upper Rank Four
A cleaner and cousin of elite magic-user En, Noi is not only extremely loyal, but she's also extremely brawny. Her size alone is a feat in itself, with Noi being one of the tallest characters in Dorehedoro, and she very easily dispatches her enemies whether they're in groups or on their own.
Noi is a typically lighthearted character but she doesn't play around with those she has an allegiance. Despite her friendly persona, she doesn't question the morally problematic work she does and seems focused on simply keeping her partner safe while she gets the job done.
2 Bondrewd, Made In Abyss, Upper Rank Three
A reprehensible character with no room for redemption, Bondrewd has experimented on children to satiate his obsession with the abyss in Made In Abyss. Other Delvers have turned a blind eye to his experiments because he's discovered so many secrets of the mysterious abyss, despite the suffering he causes.
Being a man of science who's willing to stop at nothing to achieve his goal, Bondrewd perfectly compliments the fact that Muzan's driving factor is his desire to live without the fear of death. With Muzan endlessly funding Bondrewd, and Bondrewd continuously making Muzan stronger, the pair would be nigh unbeatable.
1 King Bradley/Wrath, Fullmetal Alchemist: Brotherhood, Upper Rank Two
Easily the most skilled Homunculus when it came to hand-to-hand combat, Wrath was a genius both on the battlefield and off. He hid amongst the humans for years as lovable King Bradley, the leader of Amestris, and successfully fooled everybody long enough that he could try and carry out Father's plan.
Despite the relationships he forged in Fullmetal Alchemist: Brotherhood, Wrath was loyal to a fault to Father and showed no remorse at having to kill the entire population of Amestris, the same people he had faithfully watched over for years. To fulfill Father's desires Wrath even started multiple wars, showing he would stop at nothing to carry out his leader's wishes.