Shonen is one of the most popular anime genres. The epic battles and inspiring characters appeal to new and old fans. Yet, one of the aspects that make it so appealing is the host of terrifying villains. They may be bad, but fans love their extravagant style and dastardly plans. Most of all, they adore seeing the villains' comeuppances at the hands of the protagonists.
The global sensation Demon Slayer is about Tanjiro Kamado, whose quest to destroy the demon who killed his family brings him into contact with many other demons along the way. The despicable nature of many Shonen anime villains has led some fans to wonder how they would fare against Tanjiro in Demon Slayer.
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Isabella Is Fearfully Obedient (The Promised Neverland)
The Promised Neverland details the horrors within Grace Field House, an orphanage full of happy children, who are looked after by their Mother, Isabella. However, the two oldest children, Emma and Norman discover the orphanage is not a sanctuary but a farm.
Though she is a kind and doting caretaker, Isabella harbors the sinister knowledge of the children's fates. Even knowing that most of the children will be fed to monsters, she continues diligently working as a Mother. Isabella's troubled past and willingness to obey malicious commands would make her an asset to Muzan Kibustuji's army.
9 Mahito Enjoys Human Misery (Jujutsu Kaisen)
Mahito is a relatively young Cursed Spirit in Jujutsu Kaisen. The world is teeming with Cursed Spirits, and it is up to Yuji Itadori and his friends to dispatch them. Despite Mahito's young age, he is already a formidable opponent for Itadori and the other Jujustu Sorcerers.
Mahito's specialty is feeding off of the hatred humans feel for each other. He uses the power he gains from this malice to a devastating degree. Mahito's disregard for human life and the fact that he is already basically a demon would make him a destructive addition to the Demon Slayer universe.
8 Riz Has A Taste For Blood (Beastars)
Beastars follows the young wolf Legoshi as he battles his feelings of love and hunger for the dwarf rabbit, Haru. Additionally, there is an uneasy undercurrent that runs through their school after the brutal devouring of a herbivore student. Even with his struggles, Legoshi takes it upon himself to discover the culprit.
Unfortunately for Legoshi, the murderer happens to be his fellow theater club member, Riz. As a bear, Riz's size and overall strength would make him a powerhouse of a demon. Plus, his taste for flesh already sets him up as the perfect demon candidate.
7 Reiko Tamura's Tragic Story Is Reminiscent Of Other Demons (Parasyte)
Reiko Tamura was infected by an alien parasite in Parasyte. Though her mind has been completely taken over by the parasite conscious, she has a scientific curiosity about her new body and humanity in general. She even experiments to see if her new body can bear children.
Tamura's theory proves correct, and she gives birth to a completely human baby boy. Though she is unfeeling at first, she becomes more maternal throughout the show – even sacrificing herself for her son's sake. Like many demons, Tamura has something to gain in becoming a demon: her son's protection. Added to her already present bloodlust, she would make an interesting demon.
6 Kishins Are Born From Loss (Soul Eater)
Soul Eater is about two teenagers, Soul Evans and Maka Albarn, who are working toward upgrading Soul so he becomes a Death Scythe. As his Meister, Maka must use Soul's weapon form to eat ninety-nine Kishin souls and one witch soul to progress. However, Soul accidentally eats a cat's soul as their one-hundredth soul, causing all of their progress to be lost. The two must work their way back up if they ever want Soul to achieve his highest honor as a Weapon.
Kishins are beings created from the massive loss of pure human life. As such, they are incredibly dangerous. Like demons, Kishins possess great power that comes with a human cost. Their destructive nature and sinister beginning would make them welcome among the demons of Demon Slayer.
5 Dio Brando Thirsts For Blood (JoJo's Bizzare Adventure)
Jojo's Bizarre Adventure follows the Joestar family through time as they deal with the despicable vampire, Dio Brando, and other malicious ne'er-do-wells. Dio's original feud with Jonathan Joestar has led him on a decades-long quest for power and the destruction of the Joestar family.
Because Dio is already a vampire, he has a taste for the type of carnage demons prefer. Added to his vampirism are his immeasurable strength and ability to stop time. With all these powers combined, Dio could easily become one of Kibutsuji's Twelve Kizuki.
4 Azami Nakiri Is Full of Resentment (Food Wars!)
Food Wars! is about Souma Yukihira as he cooks his way to the top of the prestigious Totsuki Academy. Unfortunately, not everyone is impressed with Souma's unique approach to cuisine, which causes him a lot of trouble. One of Souma's opponents is the cutthroat Azami Nakiri.
Azami's quest for only one hundred percent gourmet offerings is in stark contrast to Souma's use of everyday items in his food. Azami's hatred is so great, that he begins dismantling the school he once loved. This show of pure disdain would make Nakiri an excellent demon. Plus, he cares very little for anyone but himself, much like a demon.
3 Haumea Takes Pleasure In Pain (Fire Force)
Fire Force follows Fire Force Soldier, Shinra Kusakabe. His mission is not only to rid the world of Infernals (people who have spontaneously combusted) but also to uncover the mystery of the fire that took his family. However, the sinister White Clad organization is working behind the scenes to perpetuate Infernals and ensure the rise of the Evangelist.
One of the White Clad, as well as an Adolla Burst holder, is Haumea. Her lightning abilities and enjoyment from watching others suffer would make her an ideal demon. She would give Zenitsu Agatsuma a run for his money in terms of elemental power.
2 All For One Steals Power (My Hero Academia)
My Hero Academia tracks Izuku Midoriya and class 1-A at the prestigious U.A. High School, as they train to be the best Pro Heroes Japan has to offer. However, the sinister All For One is a consistent hindrance. His power to obtain Quirks and use them for himself makes him a nearly unbeatable force.
Due to the sheer mass of his power, All For One would make a chillingly effective demon. His lack of remorse and thirst for supremacy could rival Kibutsuji's. All For One's meticulous attention to detail and disregard for life would make him a sickening demon.
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Father Has Creation In Mind (Fullmetal: Alchemist Brotherhood) 
Edward and Alphonse Elric are two brothers searching for a philosopher's stone after they committed the ultimate transmutation taboo in Fullmetal Alchemist: Brotherhood. They hope to use the stone to get Alphonse's body back from the ether. Unfortunately, there are a few who are already in possession of a philosopher's stone and are using it to devastating results.
Father is an artificially created being who seeks to become a god. He is the creator of the other Homunculi and rules over them as their leader. Like Kibutsuji, Father has created his band of followers to become the superior race among humans. Were Father to become a demon, he could very well give Kibutsuji a run for his money both in power and ambition.