The only known Demon Slayers who became demons were Kokushibo and Kaigaku. They are unquestionably the most dangerous Demons in the series due to them being able to combine their Blood Demon Art with their Breathing techniques in battle. As a result, they were ranked higher than other demons in the Demon Slayer universe. Here is a comprehensive breakdown of who these Demon Slayer-turned-demons were.
Members of the Corps would never intentionally turn into the enemy they’ve sword to destroy. However, these characters have their own motivations to do just that. Kokushibo and Kaigaku’s motivations for turning into demons were similar. While Kaigaku surrendered to become a demon to survive, Kokushibo wanted to transform into one to escape his impending death and become stronger.
Their path to being a Demon Slayer has also made them powerful enough to stand against the organization and their own former comrades, showing how strong they have become in their journey to greatness.
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Kokushibo is the Upper-Rank Moon One (1) in the Twelve Kizuki. This is the highest position next to Muzan Kibutsuji, the Demon King. He retained this position for over 500 years without faltering over other Kizukis who challenged him. He was considered to be the most dangerous Kizuki for being able to use his Breathing abilities along with his Blood Demon Art. He’s deadly enough to be called the Hashira killer. One of his feats was killing a leader of the Demon Slayer Corps which caused them to hide their future leaders.
As a Human:
Kokushibo’s human birth name was Michikatsu Tsugikuni, the older twin brother of the strongest Demon Slayer that lived, Yoriichi Tsugikuni. As a human, he pitied his brother for being deaf and for always clinging to their mother. Upon realization that Yoriichi was more powerful than him, he became filled with jealousy and hatred towards his brother.
Upon his mother’s death and Yoriichi’s departure, he became more motivated to become stronger as a samurai. He went on with his life and even had a family. However, when he met the adult Yoriichi who saved him from a demon attack, his rage toward him brewed again. This evolved into an inferiority complex and his ambition to overtake his brother’s capabilities.
- Appearance – As a human child, he resembled his twin brother’s build with spiky black hair. He has prominent eyebrows and dark purple irises. When he was a Demon Slayer Corps member, his attire consisted of a purple and black patterned kimono. His appearance as an adult further changed as he awakened his demon-slaying mark. The mark covered his forehead and his chin.
- Abilities – He was a Samurai who learned the art of the blade from a very young age. After Yoriichi’s departure from their home, he focused to become a samurai as a profession during his time. During his sudden encounter with demons who cornered him, he met Yoriichi who had become a demon slayer by then. His desire to overpower Yoriichi started again
- Breathing Style – Michikatsu’s breathing style is Moon Breathing, a direct opposite of his twin brother’s Sun breathing technique. Along with his mastery of Total Concentration Breathing, his awakened Demon Slayer mark also gave him access to view the Transparent world.
As a Demon:
Mitchikatsu was the first-ever among Demon Slayers who turned into a demon. He was pursued by Muzan after his frustrations about his fated death began to loom over him. As a result of his awakened Demon Slayer mark, it took him three days to fully become a demon. As a demon, he used Kokushibo as his name.
- Appearance – Kokushibo’s demon appearance wasn’t that much different from his original human form. The noticeable differences were the added number of eyes and their appearance. His irises were black, engraved with the kanji of his Upper Moon Rank One (1) position. The simple black spiky hair he once had now has the color red covering the tips. His nichirin blade also changed along with him. The nichirin blade now has additional protruding blades around it.
- Abilities – Being the second powerful demon next to Muzan in the franchise, he has shown incredible capabilities that would shake even a Hashira’s resolve in front of him. For nearly 500 years, he was able to gain a lot of experience and knowledge in combat against the Demon Slayers.
- Blood Demon Art – As a demon trained for combat, he was able to harness his Breathing style to complement his demon art. He was the first demon who was able to use a breathing-style technique for combat. He can create many crescent moon blades that slash in varying directions and sizes. After living for 500 years, he was able to hone his techniques and even create sub-techniques of breathing over the years he’s lived.
In the end, Kokushibo was someone who just wanted to accomplish something outside the shadow of his brother. During his last moments, he laments over his life choices and deeply regrets the path he walked through in his life.
Kaigaku is another member of the Demon Slayer Corps who turned into a demon. He replaced Daki and Gyutaro as the newly appointed Upper Moon Rank Six (6) after their death. Kokushibo was responsible for turning him into a demon after Kaigaku lost and surrendered to him. He has a strong and angry demeanor but he’s just a coward on the inside striving hard to survive.
As a Human:
Kaigaku was one of the orphans cared for by Gyomei Himejima (Stone Hashira) at his temple. He was chased out by the other kids after they caught him stealing. It was around that time when he encountered a demon who he directed to the temple to spare his own life. Later in his life, he trained with Zenitsu as a disciple of the former Thunder Hashira.
- Appearance – As a human, he had pointy black hair and turquoise-colored eyes. The manga only showed glimpses of what his appearance as a human child was like. He wore a black robe with the same hair that he had until he was an adult.
- Abilities – He was regarded as a master swordsman capable of learning the technique quickly while under the mentorship of the former Thunder Hashira. He was supposed to inherit the title alongside Zinetsu as a disciple of the former Thunder Hashira. Unfortunately, he chose to be a demon so that he could live another day.
- Breathing Style – As an aspiring Demon Slayer, he trained under the former Thunder Hashira, Jigoro Kuwajima, who taught him the fundamentals of Thunder Breathing. He was able to master five (5) of the techniques quickly, even being praised by their master. Though sadly, with his inability to master the first fundamental form of the technique, his other mastery of the techniques was useless.
As a Demon:
Kaigaku, unlike Kokushibo, lacked the necessary training to become a demon. He was a newly transformed demon who was thrown into the Upper-Rank Six (6) Kizuki position simply because he had Demon Slayer experience. However, he was completely defeated by his fellow disciple, Zenitsu, due to Kokushibo and Muzan’s overestimation of him.
- Appearance – After becoming a demon his skin became pale, and he developed fangs, elf-like ears, black pointed claws, and four black flame-like markings on both sides of his face. His eyes were engraved with his Upper Moon Rank Six (6) position in kanji and black sclera.
- Abilities – Kaikagu demonstrated immense power with his knowledge of swordsmanship, allowing him to be considered for an upper-rank position in the Kizuki. However, as a newly positioned Kizuki, he was unable to fully utilize his demon abilities.
- Blood Demon Art – Having the capabilities of a full-fledged demon, he was able to manipulate actual lightning and electricity, greatly leveling up his Thunder Breathing ability. His blood demon art is Electrokinesis. This grants him the ability to generate electricity from his flesh and blood. He uses this alongside his Thunder breathing skills.
When he was tasked to fight as the upper moon six, he was easily defeated by Zenitsu alone. This was due to him not being strong enough as a Demon Slayer and a demon. He wasn’t able to consume people that could’ve helped him gain strength as a demon.
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