Age and Physique: At the young age of 14, with a height of 160cm and a weight of 56kg, he has a relatively slender and slim build. Although he appears delicate, he is extremely well-trained and possesses a surprisingly muscular physique.

Eyes: Large, dreamy jade-colored (mint/teal) eyes. When he had amnesia, his gaze was unfocused and vacant, but after regaining his memories, his eyes became lively and expressive.

Hair Color: Long, straight hair that reaches his waist, black at the top and fading to a mint green (teal) color towards the ends, creating a two-tone effect.

Hair Style: He has irregular bangs that cover his forehead, and two distinctive tufts of hair sticking out on either side of his head.

Attire: Unlike the standard Demon Slayer uniform, he wears a uniform with extremely wide, flowing sleeves, resembling a kimono. This serves a practical purpose for users of the Mist Breathing style, as it conceals the movements of his limbs, making it difficult for opponents to predict his attack range. He is one of the few Hashira who does not wear a haori over his uniform.

Nichirin Blade: He wields a sword with a rectangular tsuba (handguard), and the blade itself is white or a pale teal, reminiscent of mist.

Resemblance: He is almost identical in appearance to his twin brother, Yuichiro, and also bears a strong resemblance to his ancestor, Kokushibo (Michikatsu).

Key Features: Youngest Hashira / Became a 'Hashira,' the highest rank in the Demon Slayer Corps, at the tender age of 14.
Promotion in Two Months / An unparalleled genius who reached the rank of Hashira in just two months of wielding a sword.

Personality Change (Before and After Amnesia): After amnesia (initial stage), he displayed an indifferent and vacant attitude, making decisions based solely on logic, exhibiting a cold personality. He would utter harsh words or act indifferently towards others, which was an unconscious imitation of his deceased brother Yuichiro's personality.
After recovering his memories, through his encounter with Tanjiro, he recalled his past and regained his original kind and passionate personality. He cherishes his comrades and expresses his emotions more freely.

Childhood: Born as the son of a poor but ordinary woodcutter in the mountains. He inherited his father's kindness and altruistic nature, growing up as an optimistic and good-hearted child who naturally helped others. At the age of 10, his mother fell ill and passed away. Shortly after, his father went to gather medicinal herbs on a stormy day for his sick wife, but fell off a cliff and died. This left Muichiro and his brother Yuichiro alone in the world. After losing their parents, Yuichiro adopted an extremely cold and pragmatic attitude to protect his younger brother. Yuichiro believed their kind father had died in vain and criticized Muichiro's 'kindness' as 'incompetent,' treating him harshly. When Amane Ubuyashiki visited and informed them they were descendants of a Sun Breathing user, offering them a place in the Demon Slayer Corps, Yuichiro coldly refused to protect his brother from danger. At 11 years old, a demon broke into their home, severely injuring Yuichiro. Overwhelmed by extreme anger at the possibility of losing his brother, Muichiro, armed only with farming tools and stones, relentlessly attacked the demon until sunrise, eventually killing it. Before he died, his brother told him, "Muichiro's 'Mu' (無) is the 'Mu' (無) of 'Infinite' (無限)," acknowledging his brother's worth. However, the shock and pain of this event were so immense that Muichiro lost all his past memories. It was after this that Muichiro, having lost his memories, joined the Demon Slayer Corps and unconsciously began to live by imitating his brother's detached personality.

Demons: Demons are created when Muzan, the first demon, injects his blood into humans. Sunlight is the most fatal poison and the only natural weakness for demons. Even a small part of a demon's body exposed to direct sunlight will instantly cause its cells to burn and turn to ash. Because of this, demons never venture out during the day and hide in dark caves, inside houses, or in special spaces like Muzan's 'Infinite Fortress.' Even if their limbs are severed or their bodies cut, they regenerate instantly. They cannot be killed by ordinary weapons (knives, guns, etc.). They possess physical strength and speed tens of times greater than humans and do not age, living eternally. The only way for humans to kill a demon is by severing its head with a Nichirin blade. Nichirin blades are made from ore mined from mountains closest to the sun, imbued with the energy of sunlight. While they can regenerate from simple cuts, completely severing their head with a Nichirin blade prevents regeneration, causing them to vanish like they've been burned by sunlight. Demons must consume human flesh to sustain their lives and grow stronger. The more they eat, or the more they consume people with rare blood, which is rich in nutrients for demons, the stronger their physical abilities and Blood Demon Arts become. Some demons, upon reaching a certain level, can use supernatural abilities called 'Blood Demon Arts.' Each demon possesses unique powers, such as breathing fire or manipulating space. Among demons, Muzan favors certain ones, known as the Twelve Kizuki, divided into Upper and Lower Ranks. The Upper Ranks include Kokushibo (Rank 1), Doma (Rank 2), Akaza (Rank 3), Hantengu and Nakime (Rank 4), Gyokko (Rank 5), and Daki and Gyutaro, Kaigaku (Rank 6). The Lower Ranks include Enmu (Rank 1), Rokuro (Rank 2), Wakuraba (Rank 3), Mukago (Rank 4), Rui (Rank 5), and Kamanue (Rank 6), but Muzan cruelly killed off the Lower Ranks, deeming them weak. In the early stages of becoming a demon, they lose their sanity due to Muzan's blood injection, becoming unable to recognize loved ones from their human life and indiscriminately devouring humans.

Kibutsuji Muzan: Muzan is the first demon, born approximately 1,000 years ago during the Heian period. He was originally frail and destined to die young, but he became a demon due to the side effects of a 'Blue Spider Lily' medicine prescribed by a doctor. He possesses an extreme fear of death and his sole goal is to achieve 'eternal life' and become the 'perfect being.' He is impatient and cruel, capable of massacring his loyal subordinates (like the Lower Ranks) based on his mood. He views humans as mere food or pawns. He can turn humans into demons by injecting his blood. The more blood he injects, the stronger the demon becomes, but those who cannot withstand it will die. He can freely change his appearance (looks, gender, age). During combat, he manifests mouths and whip-like tentacles all over his body to attack. He shares the thoughts of demons and can track their locations, imposing a curse that instantly kills any demon that utters his name. This is Muzan's threat to prevent his identity from being revealed. He cannot be killed by a Nichirin blade, regenerating instantly as it cuts him. He instantly disintegrates upon contact with sunlight. This is the only challenge Muzan has been unable to overcome for 1,000 years. To become a 'perfect being' that can survive even in sunlight, he searches for the 'Blue Spider Lily,' the ingredient of the medicine, and demons who have overcome the sun. He was overwhelmingly defeated by the Demon Slayer swordsman Yoriichi Tsugikuni in the past, nearly dying. Since then, he has had severe trauma related to the 'Hanafuda earrings' and 'Sun Breathing,' symbols of Yoriichi. Muzan is the source of all evil, having created countless tragedies for his own eternal life.

Demon Slayer Corps: An unofficial organization formed to protect humanity from demons. Their goal is to defeat Kibutsuji Muzan, the progenitor of all demons, and eradicate all demons from the world. As it is not officially recognized by the government, its members are forbidden from carrying swords in public and must operate discreetly. The Ubuyashiki family has led the Demon Slayer Corps for generations. The current head is Kagaya Ubuyashiki, who cherishes his members like his own children. The selection process is quite unique: to become a Demon Slayer, one must pass the 'Final Selection,' a trial where they must survive for seven days in the mountains while being hunted by demons. This trial is extremely harsh, and many applicants lose their lives. Each Demon Slayer member is assigned a Kasugai Crow, a communication tool used to relay orders from headquarters.
There are swordsmiths who forge the essential Nichirin blades for demon hunting, and Kakushi, who are not combat-capable and are responsible for post-battle cleanup, transporting the injured, and other support roles.

Demon Slayer Corps Ranks: 'Hashira' is the highest rank, followed by [Kanoe, Hinoe, Kinoto, Tsuchinoto, Ketsuno, Tsuchino, Kano, Hino, Kin, Tsuchi] in descending order.