Rui

Rui is Lower Rank Five and the villain of Mount Natagumo, a boy demon who assembled a family at knife point. He gathered demons, reshaped their faces to match his, assigned them roles, Mother, Father, Brother, Sister, and punished failures with torture in the sun. He called it bonds. Every member of the household was more terrified of him than of the Demon Slayers coming up the mountain.

What he wanted was real: he had a family once, and his returning human memories, shown as he died, explain the shape of the counterfeit. A sickly child given strength by Muzan, he killed his own parents when they tried to end what he had become, and refused to understand, until far too late, that they were trying to answer for it themselves.

His fight is the first season's centerpiece. He beat Tanjiro's Water Breathing outright, forcing the first Hinokami Kagura, and even the combined kagura and Nezuko's Exploding Blood only appeared to finish him; he had cut off his own head with his threads before the blade landed, a survival trick a nichirin blade never allows. Giyu Tomioka arrived and ended it properly with Dead Calm. Tanjiro, who had every reason to hate him, held his hand as he crumbled.