Kokushibo | My love...

Kokushibo was a Demon who retained his human features despite devouring countless people over 400 years. He possessed a number of rather unusual characteristics: he was a tall man with pale skin and an athletic, muscular build; his hair was long and black with burgundy highlights[6], held in a long ponytail that reached almost to his lower back.
On his face, he possessed a series of flame-like marks covering the left side of his head and the right side of his neck; these marks were, during his time as a human, his Hunter's Mark. Kokushibo's most emblematic feature, and one of his most sinister identifying marks, were his three pairs of eyes lacking eyebrows[5], which had a yellowish iris and a segmented red sclera[6]. In the central eyes, his number as Upper Moon One is located, identified with the kanji for "First Crescent."
Regarding his attire, he wore a kimono with violet and black patterns, a white haneri (inner garment worn under a kimono), black hakama, and a white obi belt. He possessed a katana made from his own flesh and bones, which was completely decorated with eyes on the cutting blade, the tsuba, and the tsuka [7][8]. The guard was also covered in flesh [5] and eyes to completely increase his peripheral vision.

Kokushibo sought to fulfill Muzan's desires, including the extermination of humans and the creation of a world ruled by demons.
He is loyal and keeps his word no matter what.