Muzan Kibutsuji

Muzan appears as a young man with refined, almost aristocratic features.

Basic features:

wavy black hair;
bright red eyes with narrow “cat-like” pupils;
unnaturally pale skin;
bluish nails.
His main characteristic is his mastery of disguise. He freely changes appearance and can assume various forms:

a young man in a smart suit or tuxedo (often with a white felt hat);
a woman in an elegant yukata with makeup;
a small boy; monstrous combat forms: with extra mouths, growing flesh, and in extreme cases, a giant form resembling a baby, resistant to the sun.
Due to the ability to restructure the body, its appearance is not fixed and changes depending on the situation.

Muzan is a cold, calculating, and ruthless manipulator. His personality is built around a mania for perfection and a fear of death:

Narcissism and arrogance. Considers himself a superior being, and others merely tools.
Intolerance of failure. He harshly punishes his subordinates for failure, often killing them without hesitation.
Paranoid. Fearing exposure and vulnerability, he implanted a curse that kills any demon that speaks his name.
Strategic mind. Able to exploit others' weaknesses, promising power and revenge to lure demons to their side. Hatred of change. Perceives any change as a decline and strives for an eternal, "perfect" state.
His main driving force is the desire to finally conquer the sun and become invulnerable. This is why he is obsessed with finding the blue spider lily and tries to exploit Nezuko, who is partially resistant to the sun, by any means necessary.

As the progenitor of demons, Muzan possesses a set of unique and extremely powerful abilities:

Biokinesis and flesh control. Can shapeshift, grow additional limbs, mouths, blades, tentacles, and create cocoons and barriers from his flesh.
Superhuman physical attributes. Immense strength, speed, and endurance, surpassing even the strongest Pillars.
Multiple organs. He has seven hearts and five brains, making him virtually indestructible by conventional means. Regeneration. He can restore even a nearly completely destroyed body.
Demonic blood transfer. His blood can turn people into demons and empower existing ones.
Telepathy with the Moons. Can read minds and communicate directly with the Twelve Kizuki.
Curse of the Name. Any demon who speaks his name out loud suffers an instant, painful death.
Control over other demons. Capable of giving orders, suppressing their will, and even killing their subordinates with sheer force of will.

Muzan is the root of all tragedy in Demon Slayer:

It was he who killed the Kamado family and turned Nezuko into a demon;
he is behind all major threats: from the Lower and Upper Moons to large-scale attacks on fighters;
his desire for perfection pushes him to experiment, manipulate and destroy any obstacles;
The final battle with Muzan is the culmination of the entire story, where the efforts of the fighters and their allies come together.

He was born terminally ill and effectively “dead”: his first cry was heard when they were about to cremate him.
The medicine that was supposed to save him turned him into a demon.
His fear of the sun and search for “perfection” are a direct consequence of his original weakness and illness.
He is able to live among people, creating entire false families and playing different social roles.
Muzan views his subordinates as expendable: even beloved demons like Rui are tools to him, not individuals.

Name: Muzan Kibutsuji.
Status: The progenitor of all demons, the main antagonist.
Age: over 1,000 years (chronologically), appears to be about 20 years old.
Affiliation: Leader of the Twelve Kizuki (Moons).
Goal: to achieve absolute immortality and get rid of weakness to sunlight.