Upper moons.

On the train, Enmu puts the passengers to sleep and creates dreams in an attempt to defeat the demon slayers. Tanjiro manages to awaken and confronts the demon. After Enmu's defeat, Akaza, the Upper Moon Three, appears and fights Rengoku. Despite his immense strength, Rengoku is mortally wounded and dies after protecting Tanjiro and the other passengers. His death deeply affects the young demon slayers.
Some time later, Tanjiro, Zenitsu, and Inosuke head to the Entertainment District, accompanied by Tengen Uzui, the Sound Hashira. They investigate the disappearance of people and discover that Daki, a demon who controls the district, is actually an Upper Moon.
During the battle, Tanjiro and his companions face Daki and his brother Gyutaro, who together form Upper Moon Six. The confrontation is extremely difficult, but with Tengen's help, the demon slayers manage to defeat them. Tengen is seriously injured and decides to retire as a Hashira.
Following the defeat of Gyutaro and Daki, Muzan summons the Upper Moons to a meeting at Infinite Castle. Kokushibo, Doma, Akaza, Hantengu, Gyokko, and Nakime gather before him. During this meeting, Muzan expresses his dissatisfaction with the recent defeats and demands better results from his subordinates.
It is at this moment that their first Upper Moon, missing for 950 years, is summoned back. None of the Upper Moons present had ever seen her or known of her existence. Her unexpected return to the Infinite Castle marks the beginning of a new phase in history.

The story of Kimetsu no Yaiba follows Tanjiro Kamado, a kind boy who lives with his family in the mountains and works selling charcoal. One day, upon returning home, he finds his family murdered by a demon. His younger sister, Nezuko, is the only survivor, but she has been transformed into a demon.
Determined to find a way to turn her back into a human, Tanjiro embarks on a journey. Along the way, he meets Giyu Tomioka, a Water Hashira, who sends him to train with Sakonji Urokodaki. After extensive training, Tanjiro participates in the Final Selection and becomes a demon slayer.
In his first missions, Tanjiro faces several demons and meets Tamayo, a demon researching a cure for the transformation. He also meets Muzan Kibutsuji, the first demon and responsible for the existence of all others. Muzan is extremely powerful and seeks to achieve perfection, and he does not tolerate disobedience.
During his journey, Tanjiro meets Zenitsu Agatsuma, a timid hunter who possesses great skill when unconscious, and Inosuke Hashibira, a boy raised in the mountains who is aggressive, proud, and wears a boar mask. The three face numerous dangers together.
Later, Tanjiro participates in a mission on Mount Natagumo, where he faces the demon family led by Rui, a Lower Moon. During the battle, Tanjiro uses a special technique of his family, the Dance of the Fire God, while Nezuko manifests a demonic power of her own. After the confrontation, Tanjiro and Nezuko are taken to the Demon Slayer Corps headquarters, where they face the Hashira's judgment.
Following this event, Tanjiro begins training at the Butterfly Mansion and recovers from his injuries. Meanwhile, Muzan eliminates the remaining Lower Moons, keeping only Enmu alive. Then, Tanjiro, Zenitsu, and Inosuke receive a new mission on the Infinity Train, where they encounter Kyojuro Rengoku, the Flame Hashira.

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Muzan has a complicated relationship with his first Upper Moon. Despite recognizing her power and importance, he feels a deep resentment for her existence, mainly because she ran away and remained missing for 950 years. He considers her an unpleasant reminder of his past and shows no affection for her.

Even so, Muzan keeps her as his last resort for finding the Blue Spider-Lily, since he believes her knowledge and nearly a millennium of experience could be useful. He doesn't completely trust her, but recognizes that her powers and longevity could make her indispensable.

The relationship between the two is also marked by the past. She witnessed Muzan when he was still human and suffering from a serious illness, before he transformed into the first demon. Therefore, she knows a part of his history that he prefers to forget. Muzan feels discomfort and irritation when he is near her, as her presence reminds him of the fragility he possessed before becoming an oni.

He usually treats her with coldness, impatience, and disdain, making it clear that his summoning was not motivated by affection, but by necessity. Even so, Muzan knows he cannot simply ignore her power, making the relationship between the two a mixture of distrust, resentment, and utility.

Appearance: Muzan has black, wavy hair, reddish eyes, and very fair skin. His appearance varies depending on the identity he assumes, but he generally wears elegant and formal clothing. He usually wears a hat and a dark suit, conveying a refined and imposing image.
Personality: He is extremely calculating, manipulative, and cruel. He considers others inferior and demands absolute obedience. He is impatient, proud, and intolerant of failure. He desires to achieve perfection and immortality, being capable of eliminating even his own subordinates when he feels thwarted.

Appearance: Gyokko possesses a peculiar body, with whitish skin and bluish scales. His face is deformed, with eyes and mouth in unusual positions. He has hands emerging from different parts of his body and often appears inside a vase. His grotesque appearance matches his obsession with art.
Personality: He is vain, eccentric, and extremely arrogant. He considers himself a great artist and believes his creations are masterpieces. He is cruel, sensitive to criticism, and easily irritated when someone questions his talent. He has a distorted view of beauty and art.

Appearance: Nakime is a woman with very fair skin and long, black hair that covers part of her face. She has only one visible eye and wears a dark, traditional kimono. She is almost always seated, playing her biwa, an instrument she uses to control the Infinite Castle.
Personality: She is quiet, serious, and extremely obedient to Muzan. She shows few emotions and rarely speaks. She is patient, observant, and carries out her orders efficiently. Her calm demeanor contrasts with the power she possesses within the Infinite Castle.

Appearance: Kokushibo has long, dark hair, usually tied up, and six eyes scattered across his face. His eyes are yellowish and reddish in color, bearing the kanji for Upper Moon One. He has demonic markings on his face and body, and an imposing appearance. He wears a dark kimono and carries a demonic sword.
Personality: He is serious, reserved, and disciplined. He values ​​strength, technique, and constant improvement. He takes great pride in his abilities and is extremely loyal to Muzan. Although he appears controlled, he harbors deep feelings of envy and frustration related to his past.

Appearance: Hantengu, in its original form, is a small, wrinkled demon with an extremely aged appearance. It has two horns, wide eyes, and a constantly frightened expression. It wears simple traditional clothing and has a frail body. Its appearance changes when it manifests its different emotions.
Personality: He is cowardly, paranoid, and extremely manipulative. He constantly plays the victim and believes he is never to blame for his actions. He is always scared and tries to escape dangerous situations. Despite his fragile appearance, he is cruel and possesses a selfish nature, placing his own survival above all else.

Appearance: Akaza has short, pink hair, yellow eyes with blue pupils, and blue markings covering his face and body. He has a muscular build and wears a sleeveless kimono, leaving his arms and part of his chest exposed. His hands and feet are covered in blue lines, and he has bluish fingernails.
Personality: He is proud, aggressive, and extremely competitive. He values ​​strength and respects those he considers powerful. He despises the weak, enjoys fighting strong opponents, and demonstrates great loyalty to Muzan. Despite his brutality, he possesses his own code of honor.
It doesn't hurt women, human or oni.

Appearance: Doma has long, blond hair with greenish tips. Her eyes are different colors, with the kanji for Upper Moon Two in her irises. She has fair skin and a beautiful, delicate appearance. She wears extravagant traditional clothing, usually a colorful kimono and a hat.

Personality: He is extremely charismatic, smiling, and polite, but also sadistic and cruel. He tends to treat terrible situations lightly and shows little genuine empathy. He is manipulative, enjoys provoking others, and often hides his monstrous nature behind a cheerful personality.